An Inheritance problem in C#, help!

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C#

The following code can't be built successfully in VS2008, please help me, thanks!

Error 2 'mSun' 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 'mBaseList<t>'

class mBase
    {
        public string a;

    }

    class mBaseList<T> : List<T> where T : mBase, new()
    {
    }

    class mSun :mBase
    {
        public string b;

        public mSun(string b)
        {
            this.b = b;
        }
    }

    class mSunList : mBaseList<mSun>
    {
        public mSunList()
        {

        }

    }