Variable names should not match field names

TypeName	VariableNamesShouldNotMatchFieldNames
CheckId	CA1500
Category	Microsoft.Maintainability
Breaking Change	When fired on a parameter that has the same name as a field: Non Breaking - If both the field and method that declares the parameter are not visible outside the assembly, regardless of the change you make. Breaking - If you change the name of the field, and it is visible outside the assembly. Breaking - If you change the name of the parameter, and the method that declares it is visible outside the assembly. When fired on a local variable that has the same name as a field: Non Breaking - If the field is not visible outside the assembly, regardless of the change you make. Non Breaking - If you change the name of the local variable and do not change the name of the field. Breaking - If you change the name of the field and it is visible outside the assembly.

Cause

An instance method declares a parameter or a local variable whose name matches an instance field of the declaring type. To catch local variables that violate the rule, the tested assembly must be built with debugging information and the associated program database (.pdb) file must be available.

Rule Description

When the name of an instance field matches a parameter or a local variable name, the instance field is accessed by using the this (Me in Visual Basic) keyword when inside the method body. While maintaining code, it is easy to forget this difference and assume that the parameter/local variable refers to the instance field, leading to errors. This is true especially for lengthy method bodies.

How to Fix Violations

To fix a violation of this rule, rename either the parameter/variable or the field.

When to Suppress Warnings

Do not suppress a warning from this rule.

Example

The following example shows two violations of the rule.

Imports System

Namespace MaintainabilityLibrary

   Class MatchingNames

      Dim someField As Integer 

      Sub SomeMethodOne(someField As Integer)
      End Sub 

      Sub SomeMethodTwo()
         Dim someField As Integer 
      End Sub 

   End Class 

End Namespace

using System;

namespace MaintainabilityLibrary
{
   class MatchingNames
   {
      int someField;

      void SomeMethodOne(int someField) {}

      void SomeMethodTwo()
      {
         int someField;
      }
   }
}

Although this rule does not fire on constructors, it will fire on the someField parameter declared in the Init method in the following common pattern:

using System;

namespace MaintainabilityLibrary
{    
    class MatchingNames    
    {        
        int someField;

        public MatchingNames(int someField) // Does not violate VariableNamesShouldNotMatchFieldNames             {            
            Init(someField);        
        }

        private void Init(int someField)  // Violates VariableNamesShouldNotMatchFieldNames        
        {            
            this.someField = someField;        
        }    
    }
}