CA1004: Generic methods should provide type parameter

Cause

The parameter signature of an externally visible generic method does not contain types that correspond to all the type parameters of the method.

Rule Description

Inference is how the type argument of a generic method is determined by the type of argument that is passed to the method, instead of by the explicit specification of the type argument. To enable inference, the parameter signature of a generic method must include a parameter that is of the same type as the type parameter for the method. In this case, the type argument does not have to be specified. When you use inference for all type parameters, the syntax for calling generic and nongeneric instance methods is identical. This simplifies the usability of generic methods.

How to Fix Violations

To fix a violation of this rule, change the design so that the parameter signature contains the same type for each type parameter of the method.

When to Suppress Warnings

Do not suppress a warning from this rule. Providing generics in a syntax that is easy to understand and use reduces the time that is required to learn and increases the adoption rate of new libraries.

Example

The following example shows the syntax for calling two generic methods. The type argument for InferredTypeArgument is inferred, and the type argument for NotInferredTypeArgument must be explicitly specified.

Imports System

Namespace DesignLibrary

   Public Class Inference

      ' This method violates the rule. 
      Sub NotInferredTypeArgument(Of T)()

         Console.WriteLine(GetType(T))

      End Sub 

      ' This method satisfies the rule. 
      Sub InferredTypeArgument(Of T)(sameAsTypeParameter As T)

         Console.WriteLine(sameAsTypeParameter)

      End Sub 

   End Class 

   Class Test

      Shared Sub Main()

         Dim infer As New Inference()
         infer.NotInferredTypeArgument(Of Integer)()
         infer.InferredTypeArgument(3)

      End Sub 

   End Class 

End Namespace

using System;

namespace DesignLibrary
{
   public class Inference
   {
      // This method violates the rule. 
      public void NotInferredTypeArgument<T>()
      {
         Console.WriteLine(typeof(T));
      }

      // This method satisfies the rule. 
      public void InferredTypeArgument<T>(T sameAsTypeParameter)
      {
         Console.WriteLine(sameAsTypeParameter);
      }
   }

   class Test
   {
      static void Main()
      {
         Inference infer = new Inference();
         infer.NotInferredTypeArgument<int>();
         infer.InferredTypeArgument(3);
      }
   }
}

Related Rules

CA1005: Avoid excessive parameters on generic types

CA1010: Collections should implement generic interface

CA1000: Do not declare static members on generic types

CA1002: Do not expose generic lists

CA1006: Do not nest generic types in member signatures

CA1003: Use generic event handler instances

CA1007: Use generics where appropriate

See Also

Reference

Generics (C# Programming Guide)