How to Resolve Error "CS0673: System.Void cannot be used from C# -- use typeof(void) to get the void type object" in C#

The Compiler Error CS0673 is a syntax restriction error. The message reads: "System.Void cannot be used from C# -- use typeof(void) to get the void type object".

In C#, void is a keyword used to specify that a method does not return a value. Under the hood, this maps to the .NET type System.Void. However, C# does not allow you to use System.Void directly in your code (as a variable type, a return type, or a class reference). The only valid use for the concept of the void type object is during Reflection (inspecting code metadata), and for that specific scenario, C# mandates a specific syntax: typeof(void).

This guide explains how to fix this error when working with Reflection or Generics.

Understanding the Restriction

Use void (lowercase keyword) when defining methods. Use typeof(void) when inspecting types via Reflection. Never use System.Void explicitly.

The compiler enforces this because System.Void is a special structure in the CLR that cannot be instantiated. You cannot have a variable of type System.Void.

Scenario 1: Reflection and Type Comparison

This is the most common use case. You are iterating over methods in a class using Reflection, and you want to check if a specific method returns void.

Example of error: attempting to compare the return type directly to the System.Void type identifier.

using System;
using System.Reflection;

public class MethodInspector
{
    public void CheckMethods()
    {
        MethodInfo[] methods = typeof(string).GetMethods();

        foreach (var m in methods)
        {
            // ⛔️ Error CS0673: System.Void cannot be used from C#.
            // C# forbids accessing the System.Void type directly like this.
            if (m.ReturnType == System.Void) 
            {
                Console.WriteLine($"{m.Name} returns nothing.");
            }
        }
    }
}

Solution: Use typeof(void)

To get the System.Type object representing void, use the typeof operator with the lowercase keyword.

Solution:

using System;
using System.Reflection;

public class MethodInspector
{
    public void CheckMethods()
    {
        MethodInfo[] methods = typeof(string).GetMethods();

        foreach (var m in methods)
        {
            // ✅ Correct: typeof(void) returns the metadata object for System.Void.
            if (m.ReturnType == typeof(void))
            {
                Console.WriteLine($"{m.Name} returns nothing.");
            }
        }
    }
}

Scenario 2: Generics and System.Void

Sometimes developers try to use System.Void as a generic type argument, often to represent a "Task that returns nothing" or a "Result object with no value."

C# does not allow void (or System.Void) as a generic argument.

Example of error

using System.Threading.Tasks;

public class Worker
{
    // ⛔️ Error CS0673 (and CS1061): You cannot use System.Void as a generic argument.
    // Developers sometimes try this when they want a generic wrapper that handles void.
    public Task<System.Void> DoWorkAsync()
    {
        return null;
    }
}

Solution: Use Task or a Unit Type

If you need a Task that returns nothing, simply use the non-generic Task.

using System.Threading.Tasks;

public class Worker
{
    // ✅ Correct: Standard Task represents a void async operation.
    public Task DoWorkAsync()
    {
        return Task.CompletedTask;
    }
}

note

Functional Programming "Unit": If you are using a library like MediatR or writing generic code (Func<T>) that requires a return type even when logical data doesn't exist, you cannot use void. The standard pattern is to define a custom struct named Unit or use bool or object as a dummy placeholder.

Conclusion

CS0673 is a syntax enforcement error.

1. Definitions: Always use the keyword void (lowercase) for method signatures.
2. Reflection: Always use typeof(void) to check if a type is void.
3. Generics: You cannot pass void or System.Void to a generic list or task. Use Task (non-generic) or a dummy Unit struct.