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How to Resolve Error "CS0148: The delegate 'delegate' does not have a valid constructor" in C#

The Compiler Error CS0148 is an initialization error. The message reads: "The delegate 'DelegateName' does not have a valid constructor".

In C#, a Delegate is a type-safe function pointer. Unlike a standard class, you cannot create an "empty" instance of a delegate using new without arguments. A delegate must point to a method (or a lambda expression) upon creation. It cannot exist in an uninitialized state where it points to nothing (except null, which is the absence of an instance).

This guide explains the constructor requirements for delegates and how to instantiate them correctly.

Understanding Delegate Constructors​

When you define a delegate:

public delegate void MyHandler(string msg);

The compiler generates a class MyHandler that inherits from System.MulticastDelegate. This generated class has a specific constructor that requires arguments (specifically, the target object and the method pointer).

It does not have a parameterless constructor. Therefore, writing new MyHandler() is syntactically invalid because no such constructor exists.

Scenario 1: Attempting Empty Instantiation​

This is the most common cause. A developer wants to declare a delegate variable and perhaps assign logic to it later, so they try to "new it up" like a standard list or object.

Example of error:

public delegate void LogDelegate(string message);

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0148: The delegate 'LogDelegate' does not have a valid constructor.
        // You cannot create a delegate that points to nowhere.
        LogDelegate logger = new LogDelegate(); 
    }
}

Solution 1: Pass a Method Name​

To fix the error, you must provide the constructor with the name of the method you want the delegate to execute.

public delegate void LogDelegate(string message);

public class Program
{
    // A method matching the delegate signature
    public static void PrintToConsole(string s)
    {
        System.Console.WriteLine(s);
    }

    static void Main()
    {
        // ✅ Correct: We pass the method name 'PrintToConsole' to the constructor.
        LogDelegate logger = new LogDelegate(PrintToConsole);
        
        logger("Hello World");
    }
}

Output:

Hello World
note

If you want the delegate to do "nothing" initially, you cannot use new Delegate(). You must assign it to null or assign it to an empty anonymous method: LogDelegate logger = delegate { };.

Solution 2: Use Method Group Conversion (Modern Syntax)​

In modern C# (2.0 and later), you rarely need to use the new DelegateType(...) syntax explicitly. You can simply assign the method name directly to the variable. The compiler handles the constructor call for you.

Solution​

public delegate void LogDelegate(string message);

public class Program
{
    public static void PrintToConsole(string s)
    {
        System.Console.WriteLine(s);
    }

    static void Main()
    {
        // ✅ Recommended: Syntax sugar. 
        // The compiler translates this to: new LogDelegate(PrintToConsole)
        LogDelegate logger = PrintToConsole;
        
        logger("Modern C#");
    }
}

Solution using Lambdas​

You can also assign a Lambda expression directly.

// ✅ Correct: Creating a delegate from an inline lambda
LogDelegate logger = (msg) => System.Console.WriteLine($"Log: {msg}");

Conclusion​

CS0148 occurs because delegates cannot be empty objects.

  1. Delegate Rule: A delegate must always point to a function (or be null).
  2. The Fix: Do not use new MyDelegate().
  3. Best Practice: Use direct assignment (MyDelegate d = MyMethod;) or Lambda expressions (MyDelegate d = x => ...;) to avoid verbose constructor syntax errors.

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