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How to Resolve Error "CS0080: Constraints are not allowed on non-generic declarations" in C#

The Compiler Error CS0080 is a syntax error regarding Generics. The message reads: "Constraints are not allowed on non-generic declarations".

In C#, the where keyword is used to define constraints on generic type parameters (e.g., where T : class). These constraints tell the compiler what kind of data is allowed to be passed into a generic placeholder. If you attempt to use a where clause on a method, class, or delegate that does not actually define a generic parameter (via <T>), the compiler throws CS0080 because there is nothing to constrain.

This guide explains the syntax rules for Generics and how to fix misplaced constraints.

Understanding Generic Syntax​

A Generic declaration must include angle brackets <T> (or other names like <TKey, TValue>) immediately after the name of the class or method. The where clause comes after the parameter list but depends entirely on the existence of those angle brackets.

  • Generic: public void Method<T>() where T : new() { } (Valid)
  • Non-Generic: public void Method() { } (Valid)
  • Invalid: public void Method() where T : new() { } (Triggers CS0080)

The compiler sees the where clause and looks for T in the method signature. If it doesn't find <T>, it assumes the method is non-generic and rejects the constraint.

Scenario 1: Forgetting the Type Parameter <T>​

This is a common typo. You intend to write a generic method that takes a parameter T, and you correctly add the constraint where T : struct, but you forget to declare <T> in the method name.

Example of error​

public class DataProcessor
{
    // ⛔️ Error CS0080: Constraints are not allowed on non-generic declarations.
    // The method 'Process' does not have <T> after its name, 
    // so 'where T : struct' has no target.
    public void Process(T input) where T : struct
    {
        // ...
    }
}

Solution: Add the Angle Brackets​

You must explicitly declare that the method is generic by adding <T>.

public class DataProcessor
{
    // ✅ Correct: Added <T> to the method signature
    public void Process<T>(T input) where T : struct
    {
        // ...
    }
}
note

If T is actually a real class name in your project (e.g., public class T {}), the error might be different (symbol resolution). CS0080 specifically happens when T is meant to be a generic placeholder.

Scenario 2: Refactoring Leftovers​

This error frequently occurs during refactoring. Imagine you had a generic class, but you decided to make it specific (e.g., changing Repository<T> to CustomerRepository). If you remove the <T> but forget to delete the where clause at the end of the line, CS0080 occurs.

Example of error​

// Original: public class Repository<T> where T : IEntity

// Refactored: We removed <T>, but forgot the 'where' clause
public class CustomerRepository where T : IEntity 
{
    // ⛔️ Error CS0080: 'CustomerRepository' is not generic, 
    // but it still has a constraint clause.
}

Solution: Remove the Constraint​

If the class is no longer generic, it cannot have constraints. Remove the where clause entirely.

// ✅ Correct: A standard non-generic class
public class CustomerRepository
{
    // Implementation specific to Customers...
}

Conclusion​

CS0080 is a syntax mismatch. You have a solution (the where clause) looking for a problem (the <T> parameter), but the parameter is missing.

  1. Check the Signature: Does your method or class name end with <T>?
  2. Add <T>: If you want it to be generic, add the angle brackets.
  3. Remove where: If you intended it to be a specific (non-generic) type, delete the constraint clause.

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