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How to Resolve Error "CS0405: Duplicate constraint 'constraint' for type parameter 'type parameter'" in C#

The Compiler Error CS0405 is a redundancy error involving Generic Constraints. The message reads: "Duplicate constraint 'ConstraintName' for type parameter 'T'".

In C#, when you define a generic class or method, you can place constraints on the type parameter (using the where keyword) to ensure it implements specific interfaces or inherits from specific classes. This error occurs simply when you list the exact same constraint twice in the same declaration. The compiler flags this as invalid syntax because the second mention adds no value and clutters the definition.

This guide explains how to identify and remove these redundant constraints.

Understanding Constraint Syntax​

Constraints define what a type T must be.

  • where T : IComparable means T must implement IComparable.
  • where T : class means T must be a reference type.

Listing a requirement once is sufficient. Listing it twice (e.g., where T : IComparable, IComparable) is a syntax error.

Scenario 1: Duplicating Interface Constraints​

This most often happens during code refactoring or merging. You might copy-paste a list of interfaces from one class to another and accidentally paste an interface that was already there.

Example of error​

using System;

// ⛔️ Error CS0405: Duplicate constraint 'IDisposable' for type parameter 'T'.
public class ResourceManager<T> where T : IDisposable, ICloneable, IDisposable
{
    // ...
}

Solution: Remove the Duplicate​

Simply delete the redundant entry from the list.

using System;

// ✅ Correct: Each constraint is listed only once.
public class ResourceManager<T> where T : IDisposable, ICloneable
{
    // ...
}

Scenario 2: Duplicating Special Keywords​

The error also applies to special constraints like class, struct, new(), or unmanaged.

Example of error​

Attempting to emphasize that a type is a class by listing it twice.

// ⛔️ Error CS0405: Duplicate constraint 'class' for type parameter 'T'.
public class EntityFactory<T> where T : class, class
{
    // ...
}

Solution: List Once​

// ✅ Correct
public class EntityFactory<T> where T : class
{
    // ...
}

Conclusion​

CS0405 is a simple housekeeping error.

  1. Check the where clause: Look at the list of constraints.
  2. Find the repetition: The error message tells you exactly which constraint (IDisposable, class, etc.) appears more than once.
  3. Delete it: Remove the extra occurrence to resolve the error.

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