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How to Resolve Error "CS0265: Partial declarations of type have inconsistent constraints for type parameter" in C#

The Compiler Error CS0265 is a consistency error related to Generic Partial Classes. The message reads: "Partial declarations of type 'Type' have inconsistent constraints for type parameter 'T'".

When defining a generic class across multiple files using the partial keyword, you can impose constraints on the generic parameter T (using the where clause). The rule is strict: if you choose to specify constraints on multiple partial declarations, those constraints must be identical. You cannot require T to be a class in one file and a struct or a specific interface in another.

This guide explains how to manage generic constraints in partial classes to avoid conflicts.

Understanding Constraint Consistency​

In C#, partial classes are merged into a single definition during compilation. The compiler cannot resolve a situation where the requirements for a generic type T differ between files.

  • File A: Says T must be a reference type (where T : class).
  • File B: Says T must be a value type (where T : struct).

Since T cannot be both, the compiler raises CS0265. Even if the constraints aren't mutually exclusive (e.g., where T : IComparable vs where T : IDisposable), they must still match exactly if they are present in both files.

Scenario 1: Conflicting Constraints​

This error occurs when two developers (or a developer and a code generator) add different requirements to the same generic class.

Example of error:

File 1: Processor.cs

namespace MyApp
{
    // Constraint: T must be a Reference Type
    public partial class Processor<T> where T : class
    {
        public void Process(T item) { }
    }
}

File 2: Processor.Extras.cs

namespace MyApp
{
    // ⛔️ Error CS0265: Partial declarations have inconsistent constraints.
    // Here, we constrain T to be a Value Type (struct).
    // It conflicts with 'class' in File 1.
    public partial class Processor<T> where T : struct
    {
        public void Reset(T item) { }
    }
}

The cleanest way to solve this—and maintain your code—is to declare the constraints on only one of the partial declarations (usually the main file). The other partial declarations automatically inherit these constraints.

Solution: remove the where clause from the secondary files.

File 1 (Main Definition):

namespace MyApp
{
    // ✅ Correct: Define constraints here.
    public partial class Processor<T> where T : class
    {
        public void Process(T item) { }
    }
}

File 2 (Secondary Definition):

namespace MyApp
{
    // ✅ Correct: No constraints listed here. 
    // The compiler implicitly applies 'where T : class' from File 1.
    public partial class Processor<T>
    {
        public void Reset(T item) { }
    }
}
note

This approach is best because if you ever need to change the constraints, you only have to edit one file.

Solution 2: Match Constraints Exactly​

If you prefer (or are required) to list constraints in all files for readability, they must be syntactically identical.

Solution: ensure both files have the exact same where clause.

File 1:

public partial class Processor<T> where T : IComparable, new()
{
}

File 2:

// ✅ Correct: Matches File 1 exactly.
public partial class Processor<T> where T : IComparable, new()
{
}
warning

If you define where T : new(), IComparable in one file and where T : IComparable, new() in the other, the compiler might accept it (semantics matter more than order for interfaces), but it is safer and cleaner to keep them identical or use Solution 1.

Conclusion​

CS0265 ensures that the rules for your generic types are consistent.

  1. Best Practice: Define generic constraints (where T : ...) in the main class file only. Leave them off other partial files.
  2. Alternative: If repeating constraints, copy and paste them so they match exactly.
  3. Check Conflicts: Ensure you haven't accidentally defined class in one file and struct in another.

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