The type 'XXX' cannot be used as type parameter 'T' in the generic type or method

Question

I have the following classes

public interface IHtmlString
{
    string ToHtmlString();
}

public class BaseClass<T> where T : BaseClass<T>, IHtmlString
{
    public BaseClass()
    {
    }
    public T Method1()
    {
        return (T)this;
    }
    public T Method2(string key, object value)
    {
        return (T)this;
    }
    public string ToHtmlString()
    {
        return ToString();
    }
}

public class DerivedClassA : BaseClass<DerivedClassA>
{
    public DerivedClassA MethodSpecificToClassA()
    {
        return this;
    }
    public override string ToString()
    {
        return "<div/>";            
    }
}

public class DerivedClassB : BaseClass<DerivedClassB>
{
    public DerivedClassB MethodSpecificToClassB()
    {
        return this;
    }
    public override string ToString()
    {
        return "<span/>";
    }
}

Note that the method returns this instance so that i can chain methods, like below

 var a = new DerivedClassA()
        .Method1()
        .MethodSpecificToClassA()
        .ToHtmlString();

However im getting compile time error

Severity Code Description Project File Line Suppression State Error CS0311 The type 'DerivedClassA' cannot be used as type parameter 'T' in the generic type or method 'BaseClass'. There is no implicit reference conversion from 'DerivedClassA' to 'System.Web.IHtmlString'. WebApplication5 C:\\MyProjects\\WebApplication5\\WebApplication5\\Models\\BaseBuilder.cs 27 Active

How do i refactor these classes so i have IHtmlString interface on base class only?

Answer 1

this worked

public class BaseClass<T> : IHtmlString where T : BaseClass<T>
{
}