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 { }