How to Resolve Error "CS0451: The 'new()' constraint cannot be used with the 'struct' constraint" in C#

The Compiler Error CS0451 is a redundancy error involving Generic Type Constraints. The message reads: "The 'new()' constraint cannot be used with the 'struct' constraint".

In C#, the struct constraint restricts the generic type T to be a non-nullable Value Type (like int, bool, or a custom struct). By definition, all value types have a public, parameterless constructor (often referred to as the default constructor) which initializes the struct to its zero-ed out state.

Because the struct constraint guarantees the existence of this constructor, adding the new() constraint (which requires a public parameterless constructor) adds no new information. The compiler considers this redundancy an error to keep constraint lists clean.

Understanding the Redundancy

When defining constraints on a generic type T:

• where T : struct: This tells the compiler that T is a value type. Value types are implicitly capable of being instantiated via new T() (which produces default(T)).
• where T : new(): This tells the compiler that T must have a public parameterless constructor.

Since all structs satisfy the requirement of new(), the struct constraint automatically implies the new() constraint. C# forbids stating the obvious in this context.

Scenario: Combining 'struct' and 'new()'

This error typically occurs when developers want to be explicit about their ability to create new instances of T, not realizing that struct already grants that permission.

Example of error:

public class Factory<T> 
    // ⛔️ Error CS0451: The 'new()' constraint cannot be used with the 'struct' constraint.
    // 'struct' already implies that T has a default constructor.
    where T : struct, new() 
{
    public T Create()
    {
        return new T();
    }
}

Solution: Remove the 'new()' Constraint

To fix the error, simply remove new() from the constraint list. You will retain the ability to instantiate the object using new T().

Solution:

// ✅ Correct: 'struct' implies the ability to use 'new T()'
public class Factory<T> where T : struct
{
    public T Create()
    {
        // This is perfectly valid because T is a struct
        return new T();
    }
}

note

Unmanaged Constraint: The same rule applies to the unmanaged constraint (introduced in C# 7.3). Because unmanaged types are strictly structs, you cannot write where T : unmanaged, new(). You must simply write where T : unmanaged.

Conclusion

CS0451 is a housekeeping rule.

1. Check Constraints: Are you using where T : struct?
2. Remove Redundancy: If yes, delete new(). It serves no purpose.
3. Functionality: Rest assured that you can still create instances of T (new T()) inside your generic class without the explicit new() constraint.