# V3194. Calling 'OfType' for collection will return an empty collection. It is not possible to cast collection elements to the type parameter.

The analyzer has detected an issue when calling 'OfType' returns an empty collection. This behavior occurs because it is not possible to cast the type of the collection items to the type on which 'OfType' filters.

Look at the example:

    public struct SyntaxToken {....}
    public class InvocationExpressionSyntax : ExpressionSyntax {....}

    public List<SyntaxToken> GetAllTokens() {....}
    public List<ExpressionSyntax> GetAllExpressions() {....}

    void ProcessInvocationExpressions()
    {
      var result = GetAllTokens().OfType<InvocationExpressionSyntax>();
      ....
    }

In 'ProcessInvocationExpressions', items of the 'InvocationExpressionSyntax' type are to be handled. For this, the collection is filtered with the 'OfType' method. However, the method is called on a collection with items of the 'SyntaxToken' type. The filtering results in an empty collection, since instances of the 'SyntaxToken' structure cannot be 'InvocationExpressionSyntax'.

In the above example, the method for getting a collection with items of the 'ExpressionSyntax' type has been mistaken for another method. 'GetAllExpressions' should be used instead of 'GetAllTokens'. The correct implementation of 'ProcessInvocationExpressions' is the following:

    void ProcessInvocationExpressions()
    {
      var result = GetAllExpressions().OfType<InvocationExpressionSyntax>();
      ....
    }

In this implementation, the collection with items of the 'ExpressionSyntax' type is filtered out. This is the base type for 'InvocationExpressionSyntax'. Therefore, 'ExpressionSyntax' can be cast to 'InvocationExpressionSyntax'. Thus, calling 'OfType' can result in a non-empty collection.

This diagnostic is classified as:

CWE-704