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How to Resolve Error "CS0752: A partial method cannot have out parameters" in C#

The Compiler Error CS0752 is a safety restriction regarding Partial Methods. The message reads: "A partial method cannot have out parameters".

In C#, standard partial methods are designed to be "optional." If the implementing part of the method is not provided, the compiler removes the method signature and all calls to it from the final binary.

This mechanism creates a conflict with out parameters. The rule for out parameters is that the method must assign a value to them. If the compiler removes the method call because it wasn't implemented, the assignment never happens. This leaves the variable uninitialized, violating the rules of definite assignment. Therefore, standard partial methods forbid out.

This guide explains how to work around this limitation using ref or by upgrading to Extended Partial Methods in C# 9.0+.

Understanding the Conflict (The Vanishing Method)​

Consider how a standard partial method works:

  1. Definition: partial void OnCalculating(out int result);
  2. Call: int x; OnCalculating(out x); Use(x);

If OnCalculating is not implemented: The compiler deletes the line OnCalculating(out x);. Now the code reads: int x; Use(x);. The variable x is used without ever being assigned a value. This causes a serious logic error. To prevent this, C# simply disallows out parameters in this context.

Scenario 1: Using out in Standard Partial Methods​

This error occurs when you define a partial hook intended to return a value, but you use the standard syntax (implicitly private, returning void).

Example of error​

public partial class Calculator
{
    // ⛔️ Error CS0752: A partial method cannot have out parameters.
    // Because implementation is optional, the compiler guarantees safety 
    // by banning 'out'.
    partial void ComputeResult(out int result);

    public void Run()
    {
        int r;
        // If ComputeResult is removed, 'r' remains unassigned.
        ComputeResult(out r); 
    }
}

Solution 1: Use ref Instead of out​

The ref keyword serves a similar purpose (passing data by reference), but it has a crucial difference: variables passed via ref must be initialized before calling the method.

If the partial method call is removed by the compiler, the variable is still safe to use because you initialized it beforehand.

public partial class Calculator
{
    // ✅ Correct: 'ref' is allowed because the caller must initialize the variable.
    partial void ComputeResult(ref int result);

    public void Run()
    {
        // We initialize it first (e.g., to 0 or -1)
        int r = 0; 
        
        // If this call is removed, 'r' is still 0. Safety is preserved.
        ComputeResult(ref r); 
        
        System.Console.WriteLine(r);
    }
}

Solution 2: Use Extended Partial Methods (C# 9.0+)​

In C# 9.0 and later (.NET 5+), the rules for partial methods were expanded. If you explicitly add an access modifier (like private, internal, or public) to the partial definition, the method becomes an Extended Partial Method.

Extended Partial Methods:

  1. Must have an implementation (it is no longer optional).
  2. Because implementation is mandatory, the call is never removed.
  3. Because the call is never removed, out parameters are allowed.
public partial class Calculator
{
    // ✅ Correct: Adding 'private' makes implementation mandatory.
    // Since it must exist, 'out' parameters are now valid.
    private partial void ComputeResult(out int result);

    public void Run()
    {
        int r;
        ComputeResult(out r); // Valid call
        System.Console.WriteLine(r);
    }
}

// In the other file:
public partial class Calculator
{
    // Implementation is REQUIRED now.
    private partial void ComputeResult(out int result)
    {
        result = 100;
    }
}
note

Standard vs. Extended:

  • partial void Name(); -> Optional implementation. No out. Returns void.
  • private partial void Name(); -> Mandatory implementation. Supports out. Can return values.

Conclusion​

CS0752 prevents uninitialized variable bugs caused by the optional nature of partial methods.

  1. If the method is optional: Use ref instead of out and initialize your variable before calling the method.
  2. If the method is mandatory: Add an access modifier (e.g., private partial) to upgrade it to an Extended Partial Method (requires C# 9.0+), which supports out parameters.

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