How to Resolve Error "CS0143: The type 'type' has no constructors defined" in C#

The Compiler Error CS0143 is a syntax and type-safety error. The message reads: "The type 'MyType' has no constructors defined".

While standard classes almost always have at least a default constructor, this error most frequently appears when working with Delegates. In modern C#, attempting to instantiate a class with the wrong arguments usually results in error CS1729. However, attempting to instantiate a Delegate with invalid arguments (like a number or a string instead of a method) specifically triggers CS0143.

This guide explains why this happens and how to properly instantiate objects and delegates.

Understanding Constructors and Delegates

• Classes: When you create a class using new, you call a Constructor. If you don't define one, C# creates a hidden, empty one: public ClassName() { }.
• Delegates: A delegate is a pointer to a method. Internally, a delegate has a constructor that accepts a Method Pointer (and a target object). It does not accept arbitrary data like integers or strings.

CS0143 typically means you tried to pass data (like 10 or "Hello") into a type (like a Delegate) that has no logic to accept data in its constructor.

Scenario 1: Invalid Delegate Instantiation (Most Common)

This is the primary cause of this specific error code in modern .NET. You define a delegate that expects to hold a reference to a function, but you try to initialize it with a value.

Example of error: you might think passing arguments to the delegate constructor passes them to the method later. This is incorrect.

public delegate void MessageHandler(string message);

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0143: The type 'MessageHandler' has no constructors defined
        // You cannot pass "Hello World" to the delegate constructor.
        // The delegate constructor expects a METHOD name, not string data.
        MessageHandler handler = new MessageHandler("Hello World");
    }
}

Scenario 2: Class Argument Mismatch

While mostly covered by error CS1729 in newer compilers, CS0143 can still occur in specific contexts where built-in types or COM objects are initialized with arguments they do not support.

Example of error: trying to pass arguments to a class that only has the default (parameterless) constructor.

public class User
{
    public string Name;
    // No constructor defined -> Default constructor User() is created.
}

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0143 (or CS1729): The type 'User' has no constructors defined
        // (that accept a string).
        var u = new User("Alice");
    }
}

Solution 1: Pass the Correct Method to Delegate

To fix the delegate error, you must pass a method name (without parentheses) to the delegate constructor, or use a lambda expression.

Correct Usage:

public delegate void MessageHandler(string message);

public class Program
{
    // A method that matches the delegate signature
    public static void PrintRed(string msg)
    {
        System.Console.WriteLine($"Red: {msg}");
    }

    static void Main()
    {
        // ✅ Correct: Pass the Method Name to the constructor
        MessageHandler handler = new MessageHandler(PrintRed);

        // ✅ Correct: Modern shorthand (Method Group Conversion)
        MessageHandler handler2 = PrintRed;

        // Invoke the delegate later with the data
        handler("Hello World");
    }
}

Solution 2: Use Object Initializers for Classes

If you got this error while trying to populate a class's data during creation, and the class does not have a custom constructor, use Object Initializer syntax.

Solution: instead of passing data in parentheses (), assign properties inside curly braces {}.

public class User
{
    public string Name { get; set; }
    public int Id { get; set; }
}

public class Program
{
    static void Main()
    {
        // ⛔️ Incorrect
        // var u = new User("Alice", 101);

        // ✅ Correct: Use Object Initializer syntax
        var u = new User 
        { 
            Name = "Alice", 
            Id = 101 
        };
    }
}

Alternatively, explicitly define a constructor in your class if you want to enforce arguments:

public class User
{
    public User(string name) { Name = name; }
    public string Name { get; set; }
}
// Now 'new User("Alice")' is valid.

Conclusion

CS0143 indicates you are misusing the new keyword.

1. Check Delegates: If instantiating a delegate, ensure you are passing a Method Name, not data values.
2. Check Classes: If instantiating a class, ensure a constructor exists for the arguments you are passing.
3. Use Initializers: If the class has no constructor, use { Property = Value } syntax instead of (Value).