Inherit from a generic base class, apply a constraint, and implement an interface in C#

You include the entire signature of your class before you define generic constraints.

class DerivedFoo<T1, T2> : ParentFoo<T1, T2>, IFoo where T2 : IBar
{
    ...
}

My recommendation: when you have a question about the syntax of the C# language, read the specification; that's why we publish it. You'll want to read section 10.1.

To answer your specific question, the order of things in a class declaration is:

attributes, in square brackets
modifiers ("public", "static", and so on)
"partial"
"class"
the class name
a comma-separated list of type parameter declarations inside angle brackets
a colon followed a comma-separated list of base types (base class and implemented interfaces, base class must go first if there is one)
type parameter constraints
the body of the class, surrounded by braces
a semicolon

Everything on that list is optional except for "class", the name, and the body, but everything must appear in that order if it appears.

public interface IFoo {}
public interface IBar {}

public class ParentFoo<T,T1> { }
public class DerivedFoo<T, T1> : ParentFoo<T, T1>, IFoo where T1 : IBar { }

public class KeyAndValue<T>
{
    public string Key { get; set; }
    public virtual T Value { get; set; }
}

public class KeyAndValue : KeyAndValue<string>
{
    public override string Value { get; set; }
}

This is an extension off the existing answers. It defaults to string if you don't supply a type. I didn't implement an interface but that shouldn't require anything different than usual.

Inherit from a generic base class, apply a constraint, and implement an interface in C#

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