public class TestBase
{
    // implementation
}
public class Test : ICollection<TestBase>
{
    // implementation
}

Somewhere else I have a property of the Test type:

public Test Test {get;set;}

How can I get the underlying type of the ICollection that the Test inherits from?

What you are probably looking for is GetGenericArguments().

var type = typeof(Test);
var collInterface = type.GetInterfaces()[0];
var generic = collInterface.GetGenericArguments()[0];

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