c# check if key exists in dictionary then pass on its value

Question

In my desktop C# application I start with a dictionary. I want to be able to check this dictionary for a key. If the dictionary has this key, I would like to pass it on to a method. If the dictionary doesn't have this key, I would like to create a blank list and just pass that on instead. How can I do this?

I get the error "given key was not present in the dictionary". Can I add a default so it is never null maybe?

// myDic was declared as a Dictionary<string, List<string>    

// Here is how I call someFunction
string text = SomeFunction(stringValue1, stringValue2, myDic[field1.field2]);

// SomeFunction looks like this
string SomeFunction (string string1, string string2, List<string> ra) 
{  
     // method 
     return stringResult;
} 

Answer 1

What you need to do is make sure the dictionary actually contains the given key in the dictionary.

If you need to extract the value by key, use TryGetValue method:

string value;
if (myDict.TryGetValue(key, out value))
{
    // Key exists in the dictionary, do something with value.
}

Answer 2

Updated based on comments. To pass one key that may or may not exist you may do this(assuming the value is a List):

// assuming the method we are calling is defined like this:
// public String SomeFunction(string string1, String string2, List<String> ra)  

List<string> valueToPassOn;
if (_ra.ContainsKey(lc.Lc))
{
     valueToPassOn = _ra[lc.Lc]
}
else
{
     valueToPassOn = new List<string>(); 
}

string text = tooltip.SomeFunction(something1, something2, valueToPassOn); 

Should you want to pass an entire dictionary (as the question originally read), regardless of whether or not the dictionary exists:

You have two options. Either create the dictionary regardless like this:

if (myDic == null)
{
     // change var and var2 to the types of variable they should be, ex:
     myDic = new Dictionary<string, List<string>>();
}
string text = SomeFunction(stringValue1, stringValue2, myDic);

or, what is probably the better option, in the declaration of the function SomeFunction add a dictionary as a variable with a default parameter. Just be sure that your function knows what to do if the dictionary is null.

string SomeFunction(string string1, string string2, Dictionary dictionary = null)
{
     // method here
}

Answer 3

You can check if the key exists using ContainsKey method and if it returns false you can pass a default value you want:

// replace default(string) with the value you want to pass
// if the key doesn't exist
var value = myDic.ContainsKey(field1.field2) ? myDic[field1.field2] : default(string);
string text = SomeFunction(stringValue1, stringValue2, value);

Answer 4

Use one of the following snippets in order to check if dictionary is empty and take some action:

var x = new Dictionary<string, string>();

if (x.Any())
{
    //....
}

if (x.ContainsKey("my key"))
{

}

if (x.ContainsValue("my value"))
{

}

if (x.Count > 0)
{

}