How to retrieve strings via reflection and concatenate it ascending-ly

Question

I have a long string that has been divided into lots of smaller Strings using the following pattern:

Public Class Test
    Public Prefix_1 as String = "1 to 100 bytes"
    Public Prefix_2 as String = "101 to 200 bytes"
    Public Prefix_3 as String = "201 to 300 bytes"
    Public Prefix_4 as String = "301 to 400 bytes"
    'and so on
End Class

And this Test class has been compiled as class library project (i.e. a .dll file) and saved to C:\Test.dll

Please note that I have no prior knowledge of how many Prefix_ string existed in the dll file.

My question is: How to retrieve all strings that start with Prefix_ via reflection and concatenate it ascending-ly (i.e. Prefix_1 & Prefix_2 ... ) into a single string?

UPDATE for bounty:

Bounty only applicable for answer in VB.NET solution

Answer 1

This ought to get you started. Sorry it's C#, but I don't remember the lambda syntax.

     Type type = Assembly.LoadFrom (@"c:\test.dll").GetType ("Test");
     object instance = type.GetConstructor (Type.EmptyTypes).Invoke (null);
     var fields = type.GetFields ().Where (f => f.Name.StartsWith ("Prefix_")).OrderBy(f => f.Name);
     string x = fields.Aggregate (new StringBuilder (), (sb, f) => sb.Append((string)f.GetValue (instance)), sb => sb.ToString ());

VB.NET

  Dim type As Type = Assembly.LoadFrom("c:\test.dll").GetType("Test")
  Dim instance As Object = Type.GetConstructor(Type.EmptyTypes).Invoke(Nothing)
  Dim fields = _
     type.GetFields() _
        .Where(Function(f) f.Name.StartsWith("Prefix_")) _
        .OrderBy(Function(f) f.Name)
  Dim bigString As String = _
     fields.Aggregate(New StringBuilder(), _
                      Function(sb, f) sb.Append(DirectCast(f.GetValue(instance), String)), _
                      Function(sb) sb.ToString())

Answer 2

If the strings are defined in the same order as in your question, you can avoid sorting, and here is a simple VB.NET answer:

Public Function Extract() As String
    Dim type As Type = Assembly.LoadFrom("C:\test.dll").GetType("YourNamespace.Test")
    Dim instance As Object = Activator.CreateInstance(type)
    Dim sb As New StringBuilder
    Dim field As FieldInfo
    For Each field In type.GetFields
        If field.Name.StartsWith("Prefix_") Then
            sb.Append(field.GetValue(instance))
        End If
    Next
    Return sb.ToString
End Function

otherwise here is a function with sorting:

Public Function Extract() As String
    Dim type As Type = Assembly.LoadFrom("c:\test.dll").GetType("YourNamespace.Test")
    Dim fields As New List(Of FieldInfo)
    Dim field As FieldInfo
    For Each field In type.GetFields
        If field.Name.StartsWith("Prefix_") Then
            fields.Add(field)
        End If
    Next

    fields.Sort(New FieldComparer)

    Dim sb As New StringBuilder
    Dim instance As Object = Activator.CreateInstance(type)
    For Each field In fields
        sb.Append(field.GetValue(instance))
    Next
    Return sb.ToString
End Function

Private Class FieldComparer
    Implements IComparer(Of FieldInfo)

    Public Function Compare(ByVal x As FieldInfo, ByVal y As FieldInfo) As Integer Implements IComparer(Of FieldInfo).Compare
        Return x.Name.CompareTo(y.Name)
    End Function
End Class

Answer 3

I'd like to propose an object oriented solution based on your answer, in Visual Basic as you requested.

Disclaimer:

Please bear in mind that i'm not a VB.NET developer. The code i provide is tested and running, but surely needs some language specific improvements.

I'm assuming that you are working with an instance of the class Test, because the fields it exposes are not Shared

Main Idea

Analyzing you requirements I found that is important to:

• Centralize the naming strategy on of the fields in one place in order for your solution to be mantenible
• Take care of the sorting of the fields. If you have Prefix_1, Prefix_2 and Prefix_11 you can get then sorted in a wrong way: Prefix_1, Prefix_11 and Prefix_2.
• Validate there are no missing field names (i.e. jump from Prefix_1 to Prefix_3)

From what you asked, i modeled each of the fields holding the chunks of the string in a class named StringChunkField. This class models each a prefixed field holding a chunk of the string and has the following responsibilities:

• Provide information about the field itself: name, number, chunk of the string that holds and the number of characters in it
• Centralize information about the format and numbering used to name the fields. Here is defined the prefix to look for, and the number at with the field's names starts.
• From the previous item, it can answer whether a field is the one beginnig a string or not, and whether a field is a StringChunkField or not.

• Implements IComparable to centralize sorting logic in one place (it's based on the field number)

  Imports System.Reflection
  
  Friend Class StringChunkField
      Implements IComparable(Of StringChunkField)
  
      #Region "Fields"
      Private ReadOnly _number As Integer
      Private _name As String
      Private _stringChunk As String
      Private Shared _beginningOfStringFieldNumber As Integer = 1
      Private Shared _namePrefix As String = "Prefix_"
  
      #End Region
  
      Public Sub New(ByRef field As FieldInfo, ByRef target As Object)
          _name = field.Name
          _stringChunk = field.GetValue(target)
          _number = ExtractFieldNumber(field.Name)
      End Sub
  
      #Region "Properties"
  
      ' Returns the field's number
      Public ReadOnly Property Number() As Integer
          Get
              Return _number
          End Get
      End Property
  
      ' Returns the field's name (includes the number also)
      Public ReadOnly Property Name() As String
          Get
              Return _name
          End Get
      End Property
  
      ' Returns the chunk of the string this fields holds
      Public ReadOnly Property StringChunk() As String
          Get
              Return _stringChunk
          End Get
      End Property
  
      ' Returns the number of characters held in this field
      Public ReadOnly Property NumberOfCharacters() As Integer
          Get
              If (String.IsNullOrEmpty(StringChunk)) Then
                  Return 0
              Else
                  Return StringChunk.Length
              End If
          End Get
      End Property
  
      Public Shared ReadOnly Property BeginningOfStringFieldNumber() As String
          Get
              Return _beginningOfStringFieldNumber
          End Get
      End Property
  
      #End Region
  
      #Region "Comparison"
  
      Public Function CompareTo(ByVal other As StringChunkField) As Integer Implements IComparable(Of StringChunkField).CompareTo
          Return Number.CompareTo(other.Number)
      End Function
  
      Function IsFollowedBy(ByVal other As StringChunkField) As Object
          Return other.Number = Number + 1
      End Function
  
      #End Region
  
      #Region "Testing"
  
      Public Function HoldsBeginingOfTheString() As Boolean
          Return Number = 1
      End Function
  
      Public Shared Function IsPrefixField(ByVal field As FieldInfo) As Boolean
          Return field.Name.StartsWith(_namePrefix)
      End Function
  
      #End Region
  
      Private Function ExtractFieldNumber(ByVal fieldName As String) As Integer
          Dim fieldNumber As String = fieldName.Replace(_namePrefix, String.Empty)
          Return Integer.Parse(fieldNumber)
      End Function
  End Class
  

Now we have defined what is a StringChunkField, what name prefix to use and how to build one, we can query an object for the string it contains with an instance of the TypeEmbeddedStringReader class.

The responsibilities for it are:

• Find all the StringChunkFields presents in an object
• Validate whether the numbering of the fields found starts according to the base defined in StringChunkField and if the numbers are consecutive

• Rebuild the embedded string in the object from the StringChunkFields values

  Imports System.Reflection
  Imports System.Text
  
  Public Class TypeEmbeddedStringReader
  
      Public Shared Function ReadStringFrom(ByRef target As Object) As String
          ' Get all prefix fields from target
          ' Each StringChunkField hold a chunk of the String to rebuild
          Dim prefixFields As IEnumerable(Of StringChunkField) = GetPrefixFieldsFrom(target)
  
          ' There must be, at least, one StringChunkField
          ValidateFieldsFound(prefixFields)
          ' The first StringChunkField must hold the beggining of the string (be numbered as one)
          ValidateFieldNumbersBeginAtOne(prefixFields)
          ' Ensure that no StringChunkField number were skipped
          ValidateFieldNumbersAreConsecutive(prefixFields)
  
          ' Calculate the total number of chars of the string to rebuild to initialize StringBuilder and make it more efficient
          Dim totalChars As Integer = CalculateTotalNumberOfCharsIn(prefixFields)
          Dim result As StringBuilder = New StringBuilder(totalChars)
  
          ' Rebuild the string
          For Each field In prefixFields
              result.Append(field.StringChunk)
          Next
  
          ' We're done
          Return result.ToString()
      End Function
  
  #Region "Validation"
  
      Private Shared Sub ValidateFieldsFound(ByVal fields As List(Of StringChunkField))
          If (fields.Count = 0) Then Throw New ArgumentException("Does not contains any StringChunkField", "target")
      End Sub
  
      Private Shared Sub ValidateFieldNumbersBeginAtOne(ByVal fields As List(Of StringChunkField))
          ' Get the first StringChunkField found
          Dim firstStringChunkField As StringChunkField = fields.First
  
          ' If does not holds the begining of the string...
          If (firstStringChunkField.HoldsBeginingOfTheString() = False) Then
              ' Throw an exception with a meaningful error message
              Dim invalidFirstPrefixField = String.Format("The first StringChunkField found, '{0}', does not holds the beggining of the string. If holds the beggining of the string, it should be numbered as '{1}'.", firstStringChunkField.Name, StringChunkField.BeginningOfStringFieldNumber)
              Throw New ArgumentException(invalidFirstPrefixField, "target")
          End If
      End Sub
  
      Private Shared Sub ValidateFieldNumbersAreConsecutive(ByVal fields As List(Of StringChunkField))
          For index = 0 To fields.Count - 2
              ' Get the current and next field in fields
              Dim currentField As StringChunkField = fields(index)
              Dim nextField As StringChunkField = fields(index + 1)
  
              ' If the numbers are consecutive, continue checking
              If (currentField.IsFollowedBy(nextField)) Then Continue For
  
              ' If not, throw an exception with a meaningful error message
              Dim missingFieldMessage As String = String.Format("At least one StringChunkField between '{0}' and '{1}' is missing", currentField.Name, nextField.Name)
              Throw New ArgumentException(missingFieldMessage, "target")
          Next
      End Sub
  
  #End Region
  
      Private Shared Function CalculateTotalNumberOfCharsIn(ByVal fields As IEnumerable(Of StringChunkField)) As Integer
          Return fields.Sum(Function(field) field.NumberOfCharacters)
      End Function
  
      Private Shared Function GetPrefixFieldsFrom(ByVal target As Object) As List(Of StringChunkField)
          ' Find all fields int the target object
          Dim fields As FieldInfo() = target.GetType().GetFields()
          ' Select the ones that are PrefixFields 
          Dim prefixFields As IEnumerable(Of StringChunkField) = From field In fields Where StringChunkField.IsPrefixField(field) Select New StringChunkField(field, target)
          ' Return the sorted list of StringChunkField found
          Return prefixFields.OrderBy(Function(field) field).ToList()
  
      End Function
  End Class
  

Usage

I prepared some sample types to test the behavior of the TypeEmbeddedStringReader class and the way to use. Simply, you have to call the Shared function ReadStringFrom passing as argument an object containing the string to read from.

Here are the sample types:

    Public Class SampleType
        Public Prefix_1 As String = "1 to 100 bytes"
        Public Prefix_2 As String = "101 to 200 bytes"
        Public Prefix_3 As String = "201 to 300 bytes"
        Public Prefix_4 As String = "301 to 400 bytes"
    End Class

    Public Class TypeWithoutString

    End Class

    Public Class TypeWithNonConsecutiveFields
        Public Prefix_1 As String = "1 to 100 bytes"
        Public Prefix_5 As String = "101 to 200 bytes"
    End Class

    Public Class TypeWithInvalidStringBeginning
        Public Prefix_2 As String = "1 to 100 bytes"
    End Class

Here is the main Module i used to test it:

    Imports TypeEmbeddedStringReader.Samples

    Module Module1
        Sub Main()
            ExtractStringFrom(New TypeWithoutString())
            ExtractStringFrom(New TypeWithInvalidStringBeginning())
            ExtractStringFrom(New TypeWithNonConsecutiveFields())
            ExtractStringFrom(New SampleType())
        End Sub

        Private Sub ExtractStringFrom(ByVal target As Object)
            Try
                Dim result As String = TypeEmbeddedStringReader.ReadStringFrom(target)
                Console.WriteLine(result)
            Catch exception As ArgumentException
                Console.WriteLine("Type '{0}': {1}", target.GetType(), exception.Message)
            End Try
            Console.WriteLine()
        End Sub
    End Module

And the results from running it:

    Type 'TypeEmbeddedStringReader.Samples.TypeWithoutString': Does not contains any StringChunkField
    Parameter name: target

    Type 'TypeEmbeddedStringReader.Samples.TypeWithInvalidStringBeginning': The first StringChunkField found, 'Prefix_2', does not holds the beggining of the string. If holds the beggining of the string, it should be numbered as '1'.
    Parameter name: target

    Type 'TypeEmbeddedStringReader.Samples.TypeWithNonConsecutiveFields': At least one StringChunkField between 'Prefix_1' and 'Prefix_5' is missing
    Parameter name: target

    1 to 100 bytes101 to 200 bytes201 to 300 bytes301 to 400 bytes

Please let me know if worked for you and if i can be of any other help to you.

Update

As requested by Gens, i added a function to the TypeEmbeddedStringReader class to read a string from an instance of a type providing it's name and assembly file:

    Public Shared Function ReadStringFromInstanceOf(ByRef assemblyFile As String, ByRef targetTypeName As String)
        Dim assembly As Assembly = assembly.LoadFrom(assemblyFile)
        Dim targetType As Type = assembly.GetType(targetTypeName)

        Dim target As Object = Activator.CreateInstance(targetType)

        Return ReadStringFrom(target)
    End Function

Here is the sample type i used for testing:

    Public Class UnorderedFields
        Public Prefix_2 As String = "101 to 200 bytes"
        Public Prefix_4 As String = "301 to 400 bytes"
        Public Prefix_1 As String = "1 to 100 bytes"
        Public Prefix_3 As String = "201 to 300 bytes"
    End Class

Here is the code that tests it:

    Dim assemblyFile As String = Assembly.GetExecutingAssembly()
    Dim targetTypeName As String = "TypeEmbeddedStringDemo.UnorderedFields"
    Console.WriteLine(TypeEmbeddedStringReader.ReadStringFromInstanceOf(assemblyFile, targetTypeName))

This is the output from the code above:

    1 to 100 bytes101 to 200 bytes201 to 300 bytes301 to 400 bytes

I hope this helped you to solve your problem. Please tell me if you need anything else!

Update 2

Answering to Gens, the reason why Simon's solution is not working is because the comparison is being done on the field name. The following example fails in its ordering (just to show the sorting problem, besides it's invalid)

    Public Class UnorderedFields
        Public Prefix_2 As String = "101 to 200 bytes"
        Public Prefix_11 As String = "301 to 400 bytes"
        Public Prefix_1 As String = "1 to 100 bytes"
        Public Prefix_3 As String = "201 to 300 bytes"
    End Class

It gives:

    1 to 100 bytes**301 to 400 bytes**101 to 200 bytes201 to 300 bytes

Fixing the comparer's implementation to use numbers instead of names:

    Public Function Compare(ByVal x As FieldInfo, ByVal y As FieldInfo) As Integer Implements IComparer(Of FieldInfo).Compare
        Dim xNumber = Integer.Parse(x.Name.Replace("Prefix_", String.Empty))
        Dim yNumber = Integer.Parse(y.Name.Replace("Prefix_", String.Empty))
        Return xNumber.CompareTo(yNumber)
    End Function

Gives the right result:

    1 to 100 bytes101 to 200 bytes201 to 300 bytes301 to 400 bytes

Hope it helps.

Answer 4

You have public fields so, From the Type object representing the class get the FieldInfo objects and exclude those whose name doesn't start with Prefix_

Once you have those you can then call GetValue on the FieldInfo objects with the object (your instance of class Test) as the parameter to get the value of the field.

If you need to order the results in anyway, then I'd suggest a LINQ statement

Sorry, I don't know VB otherwise I'd write you some code.

UPDATE: Some C# code

Test myTestInstance = ... // Do stuff to the the instance of your class
Type myType = typeof(Test); // Or call GetType() on an instance
FieldInfo[] myFields = myType.GetFields();
var myPrefixedFields = myFields
                         .Where(fi => fi.Name.StartsWith("Prefix_"))
                         .OrderBy(fi => fi.Name);
string result = string.Empty;
foreach(FieldInfo fi in myPrefixedFields)
{
    // You may prefer to use a string builder.
    result += fi.GetValue(myTestInstance);
}

That should be about it.

Answer 5

Got it in C# code (VB.NET is a bit rusty :)):

using System;
using System.Linq;
using System.Text;
using System.Reflection;

void ExtractFields()
{
        const string prefix = "Prefix_";
        Assembly assembly = Assembly.LoadFile("C:\\Test.dll");
        Type classTestType = assembly.GetType("Test");
        var classTest = Activator.CreateInstance(classTestType);
        FieldInfo[] fields = classTestType.GetFields(BindingFlags.GetField)
            .Where(m => m.Name.StartsWith(prefix))
            .OrderBy(m => m.Name)
            .ToArray();
        var sb = new StringBuilder();
        foreach (FieldInfo field in fields)
        {
            sb.Append(field.GetValue(classTest));
        }
        string allStringConcatenated = sb.ToString();
}

Answer 6

Using a test class, slightly modified, from your question:

Public Class Test
  Public Prefix_15 As String = "501 to 600 bytes"
  Public Prefix_5 As String = "401 to 500 bytes"
  Public Prefix_1 As String = "1 to 100 bytes"
  Public Prefix_2 As String = "101 to 200 bytes"
  Public Prefix_3 As String = "201 to 300 bytes"
  Public Prefix_4 As String = "301 to 400 bytes"
End Class

Running the following function:

Public Function GetPrefixString() As String
  Dim type As Type = Assembly.LoadFrom("C:\test.dll").GetType("Test.Test")
  Dim test As Object = Activator.CreateInstance(type)

  Dim fieldList As New List(Of String)
  For Each field As FieldInfo In _
                    From x In type.GetFields _
                    Where x.Name.StartsWith("Prefix_") _
                    Order By Convert.ToInt32(x.Name.Replace("Prefix_", String.Empty))
    fieldList.Add(field.GetValue(test))
  Next

  Return String.Join(String.Empty, fieldList.ToArray)
End Sub

yields the following results:

1 to 100 bytes101 to 200 bytes201 to 300 bytes301 to 400 bytes401 to 500 bytes501 to 600 bytes