How to Resolve Error "CS0670: Field cannot have void type" in C#

The Compiler Error CS0670 is a type definition error. The message reads: "Field cannot have void type".

In C#, void is a special keyword used exclusively to specify the return type of a method that does not return a value. It is not a data type that can hold a value. Since a Field (a class-level variable) exists specifically to store data, declaring a field as void is logically impossible because "nothing" cannot be stored.

This guide explains why this error happens and how to fix your field definitions.

Understanding the Void Keyword

• Valid Use: public void MyMethod() { } (This method performs an action and returns nothing).
• Invalid Use: public void MyVariable; (Create a storage box that holds nothing).

Since a field is a storage location in memory (e.g., 4 bytes for an int, 8 bytes for a long), the compiler must know what kind of data to put there. void has no size and no representation in memory storage.

Scenario: The Invalid Declaration

This error often occurs due to copy-paste mistakes (copying a method signature but forgetting the parentheses) or a misunderstanding of how to represent "empty" data.

Example of error: copying a method definition but trying to make it a field.

public class DataHandler
{
    public int Id;

    // ⛔️ Error CS0670: Field cannot have void type.
    // The developer might have meant to write a method 'ProcessData',
    // or meant to use a different type.
    public void ProcessData; 
}

Solution: Use a Valid Data Type

If you intended to store data, change void to a real type. If you intended to define an action, convert the field into a method or a delegate.

Fix 1: Change to a Type (Variable)

If you just want a generic placeholder, use object.

public class DataHandler
{
    // ✅ Correct: Using a valid type
    public object ProcessData; 
}

Fix 2: Convert to Method (Action)

If you meant to define a function, add parentheses.

public class DataHandler
{
    // ✅ Correct: Added '()' to make it a method
    public void ProcessData()
    {
        // Code here
    }
}

Fix 3: Use a Delegate (Function Pointer)

If you want a field that holds a reference to a void method (so you can assign different functions to it), use the Action delegate.

using System;

public class DataHandler
{
    // ✅ Correct: 'Action' is a type that points to a 'void' method.
    public Action ProcessData; 
}

// Usage:
// handler.ProcessData = () => Console.WriteLine("Processing");

Conclusion

CS0670 prevents you from creating variables that cannot hold values.

1. Check the Syntax: Did you forget the () on a method declaration?
2. Check the Intent: Do you want to store a function? Use Action or Delegate, not void.
3. Check the Type: Do you want to store data? Use int, string, bool, etc.