C# 3.5 partial class String IsNullOrWhiteSpace

Question

I'm trying to create extra functionality to the String class (IsNullOrWhitespace as in .NET4 ) But I'm having an problem with referencing:

Error 1 'String' is an ambiguous reference between 'string' and 'geolis_export.Classes.String'

I don't want to create an extension method. Because this will crash if string x = null;

Usage:

private void tbCabineNum_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
    e.Handled = !e.Text.All(Char.IsNumber) || String.IsNullOrWhiteSpace(e.Text);
}

String partial:

public partial class String
{
    public static bool IsNullOrWhiteSpace(string value)
    {
        if (value == null) return true;
        return string.IsNullOrEmpty(value.Trim());
    }
}

Is it not possible to create extras for the String class? I have tried to put the partial in the System namespace, but this gives other errors.

Renaming String to String2 fixes the problem also. But this is not what I want, because then there is no reference with the original String class.

Answer 1

It is not possible like this, because the string class in the .NET framework is not partial.
Instead, use a real extension method like this:

public static class StringExtensions
{
    public static bool IsNullOrWhiteSpace(this string value)
    {
        if (value == null) return true;
        return string.IsNullOrEmpty(value.Trim());
    }
}

The usage would then be like this:

string s = "test";
if(s.IsNullOrWhiteSpace())
    // s is null or whitespace

As with all extension methods, the call will not result in a null reference exception if the string is null:

string s = null;
if(s.IsNullOrWhiteSpace()) // no exception here
    // s is null or whitespace

The reason for this behavior is that the compiler will translate this code into IL code that is equivalent to the IL code of the following:

string s = null;
if(StringExtensions.IsNullOrWhiteSpace(s))
    // s is null or whitespace

Answer 2

An extension method has to be defined as a static method inside a static class. Also notice the this keyword on the parameter.

public static class MyExtensions
{
    public static bool IsNullorWhitespace(this string input)
    {
         // perform logic
    }
}

What you have done by omitting the static on the class is define a competing class within your assembly, hence the ambiguous message from the compiler.