The given key was not present in the dictionary - what it means, and how to avoid it

A common error posted on the CRM Development forum is ‘the given key was not present in the dictionary’. This is a relatively easy error to diagnose and fix, provided you know what it means. It will also help to identify the line in the code at which the error occurs, which is most easily determined by debugging.

The error refers to a ‘dictionary’, and a ‘key’. The ‘dictionary’ is a type of collection object (i.e. it can contain many values), and the ‘key’ is the means by which you specify which value you want. The following two lines of code both show an example:
Entity e = context.InputParameters["Target"];
string name = e.Attributes["name"];  // Note that this is equivalent to: string name = e["name"];
In the first line, InputParameters is the dictionary, and "Target" is the key. In the second line, Attributes is the dictionary, and "name" is the key. The error ‘The given key is not present in the dictionary’ simply means that the dictionary does not have a value that corresponds to the key. So, this error would occur in the first line if InputParameters does not contain a key called "Target", or in the second line if there were no "name" in Attributes.

The way to avoid these errors is simple; test if the key exists before trying to use it. Different collection classes can provide different ways to perform this test, but the collection classes in the CRM SDK assemblies all inherit from an abstract DataCollection class that exposes a Contains method, so you can use a consistent approach across these collection classes.
if (context.InputParameters.Contains("Target"))
{
 Entity e = context.InputParameters["Target"];
 if (e.Attributes.Contains("name"))
 {
  string name = e.Attributes["name"];
 }
}

There are a few common reasons of the use of CRM collection classes where a key might not be present when you expect it:

• Within a plugin, the values in context.InputParameters and context.OutputParameters depend on the message and the stage that you register the plugin on. For example, "Target" is present in InputParameters for the Create and Update messages, but not the SetState message. Also, OutputParameters only exist in a Post stage, and not in a Pre stage. There is no single source of documentation that provides the complete set of InputParameters and OutputParameters by message and stage, though this post provides a list of the most common ones for CRM 4, and most of these still apply in CRM 2011
• The Attributes collection of an Entity will only contain values for attributes that have a value. You may get the Entity from a Retrieve or RetrieveMultiple having specified a ColumnSet with the attribute you want, but this attribute will not be present in the Attributes collection if there were no data in that attribute for that record
• Within a plugin, the Attributes collection of an Entity that you obtain from the "Target" InputParameter will only contain attributes that were modified in the corresponding Create or Update method. Using the example above, if this were in a plugin registered on the Update message, the "name" attribute would only be present if the "name" attribute was changed as part of the Update; the "Target" InputParameter will not contain all the attributes for the entity