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How to Resolve Error "CS0692: Duplicate type parameter 'identifier'" in C#

The Compiler Error CS0692 is a naming collision error specific to Generic Definitions. The message reads: "Duplicate type parameter 'identifier'".

In C#, when defining a generic class, interface, method, or delegate, you define placeholders (Type Parameters) inside angle brackets < >. Each placeholder acts as a unique variable representing a Type. Just as a method cannot have two parameters named x (e.g., void Add(int x, int x)), a generic definition cannot have two type parameters with the same name (e.g., class Data<T, T>). The compiler requires unique identifiers to distinguish them.

This guide explains how to identify and resolve these duplicate generic names.

Understanding Generic Type Parameters​

Type parameters allow you to create flexible code templates.

  • Definition: public class Map<TKey, TValue>
  • Usage: TKey is used for keys, TValue for values.

If you write public class Map<T, T>, and then write a field public T data;, the compiler does not know which T you are referring to. To avoid this ambiguity, unique names are strictly enforced within the angle brackets.

Scenario 1: Duplicates in Class Definitions​

This usually happens when copy-pasting code or when a developer mistakenly thinks they need to declare the parameter twice to use it in two places.

Example of error​

Attempting to define a class that takes two generic arguments, but giving them the same identifier.

// ⛔️ Error CS0692: Duplicate type parameter 'T'
public class Pair<T, T>
{
    public T First { get; set; }
    public T Second { get; set; }
}

Solution: Rename or Remove​

Option A: Rename​

If the types are distinct (e.g., a Key and a Value), give them descriptive, unique names.

// ✅ Correct: TKey and TValue are distinct identifiers.
public class Pair<TKey, TValue>
{
    public TKey Key { get; set; }
    public TValue Value { get; set; }
}

Option B: Remove​

If both properties are meant to be the same type (e.g., a coordinate point where X and Y are both int), you only need to define the generic parameter once. You can then use that single T multiple times inside the class.

// ✅ Correct: Define 'T' once, use it multiple times.
public class Pair<T>
{
    public T First { get; set; }
    public T Second { get; set; }
}
note

Naming Convention: It is standard practice to prefix generic parameters with T (e.g., T, TItem, TResult).

Scenario 2: Duplicates in Method Definitions​

The same rule applies to generic methods. You cannot reuse a name in the method's type parameter list.

Example of error​

public class Converter
{
    // ⛔️ Error CS0692: Duplicate type parameter 'T'
    public void Convert<T, T>(T input) 
    {
    }
}

Solution​

Rename the parameters to reflect their specific roles (e.g., Input vs. Output).

public class Converter
{
    // ✅ Correct: 'TInput' and 'TOutput' are unique.
    public TOutput Convert<TInput, TOutput>(TInput input)
    {
        return default(TOutput);
    }
}

Conclusion​

CS0692 is a simple naming conflict check.

  1. Check the Brackets: Look inside <...> in your class or method definition.
  2. Find the Duplicate: Did you type <T, T>?
  3. Fix:
    • If you need two different types, rename them (<T1, T2>).
    • If you only need one type used multiple times, delete the duplicate declaration (<T>).

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