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How to Resolve Error "CS0310: The type 'typename' must be a non-abstract type with a public parameterless constructor" in C#

The Compiler Error CS0310 is a generic constraint violation error. The message reads: "The type 'TypeName' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'GenericClass<T>'".

In C#, generic definitions can enforce constraints on their type parameters using the where keyword. The constraint where T : new() specifically tells the compiler: "I intend to create instances of T using new T() inside this class/method."

If you try to use a type that cannot be instantiated (because it is abstract) or does not have a public empty constructor (so new T() would fail), the compiler raises CS0310 to prevent a runtime crash.

Understanding the new() Constraint​

When you define a generic class like this:

public class Factory<T> where T : new()
{
    public T CreateInstance()
    {
        // The constraint allows this line to compile:
        return new T(); 
    }
}

Any type you pass to Factory<T> must satisfy two rules:

  1. It must be a concrete class (not abstract, not an interface).
  2. It must have a public constructor that takes zero arguments (public ClassName()).

Scenario 1: Missing Parameterless Constructor​

This is the most common cause. If you define a constructor that takes arguments (e.g., public User(string name)), the compiler stops generating the default parameterless constructor.

Example of error​

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

    // Defining this removes the default 'User()' constructor
    public User(string name)
    {
        Name = name;
    }
}

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0310: 'User' must have a public parameterless constructor 
        // to satisfy the 'new()' constraint in 'Factory<T>'.
        Factory<User> userFactory = new Factory<User>(); 
    }
}

Solution: Add the Constructor​

Explicitly add a public, empty constructor to the class.

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

    public User(string name)
    {
        Name = name;
    }

    // ✅ Correct: Added parameterless constructor to satisfy constraint
    public User() { }
}

public class Program
{
    static void Main()
    {
        // ✅ Correct: Now valid
        Factory<User> userFactory = new Factory<User>(); 
    }
}

Scenario 2: Using Abstract Classes​

You cannot create an instance of an abstract class (new AbstractClass() is illegal). Therefore, you cannot pass an abstract class to a generic that requires new().

Example of error​

public abstract class BaseEntity 
{ 
    public BaseEntity() { }
}

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0310: 'BaseEntity' must be a non-abstract type.
        Factory<BaseEntity> entityFactory = new Factory<BaseEntity>();
    }
}

Solution: Use a Concrete Derived Class​

You must pass a concrete class that inherits from the abstract base.

public class Customer : BaseEntity
{
    // Inherits BaseEntity logic but can be instantiated
}

public class Program
{
    static void Main()
    {
        // ✅ Correct: Customer is concrete
        Factory<Customer> customerFactory = new Factory<Customer>();
    }
}

Scenario 3: Private or Protected Constructors​

Even if the parameterless constructor exists, it must be public. If it is private (common in Singleton patterns) or protected, the generic class cannot access it to call new T().

Example of error​

public class SingletonService
{
    // Private constructor prevents external instantiation
    private SingletonService() { }
}

public class Program
{
    static void Main()
    {
        // ⛔️ Error CS0310: The parameterless constructor is not public.
        Factory<SingletonService> service = new Factory<SingletonService>();
    }
}

Solution​

If you control the class, make the constructor public. If you cannot make it public (because of design patterns), you cannot use it with a generic that enforces where T : new(). You would need to redesign the generic to accept a factory delegate (Func<T>) instead of using the new() constraint.

Alternative Design (Removing Constraint):

// Remove 'where T : new()'
public class FlexibleFactory<T>
{
    private Func<T> _creator;

    public FlexibleFactory(Func<T> creator)
    {
        _creator = creator;
    }

    public T Create() => _creator();
}

// Usage:
// var f = new FlexibleFactory<SingletonService>(() => SingletonService.Instance);

Conclusion​

CS0310 ensures that new T() will work at runtime.

  1. Check the Constructor: Does the class you are passing have public ClassName() { }?
  2. Check Abstract: Are you trying to pass an abstract class or interface? Pass a concrete implementation instead.
  3. Check Visibility: Is the constructor private? It must be public.

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