static void Main(string[] args)
{
    Func<int, int, string> func = Sum;

    string result = func(2, 4);
    Console.WriteLine(result); //Print Sum: 6
    
    // I can also call, easier. Why I need here this Func<int, int, string> func = Sum;
    int result = Sum(7,8)
}

public static string Sum(int a, int b)
{
    string str = string.Format("Sum: {0}", (a + b));
    return str;
}

// The same

static void Main(string[] args)
{
    // return value is string,     Transfer parameters 1 (int) and 2 (int)
    Func<int, int, string> func = (a, b) => string.Format("Sum: {0}", (a + b));

    string result = func(2, 4);
    Console.WriteLine(result); //Print Sum: 6
}


// ###############################
// ## I make more samples.
private void btnFuncTest_Click(object sender, EventArgs e)
{

    Func<int, int, string> func = Sum;

    string result = func(2, 4);
    Debug.WriteLine("Simple func Sum= " + result); //Print Sum: 6

    Func<int, int, string> func1 = delegate (int a, int b)
    {
        string str = string.Format("Sum: {0}", (a + b));
        return str;
    };

    string result1 = func(2, 4);
    Debug.WriteLine("delegate func Sum= " + result1); //Print Sum: 6

    // --------------------

    Func<int, int, string> func2 = (a, b) => string.Format("Sum: {0}", (a + b));

    string result2 = func2(2, 4);
    Debug.WriteLine("Lamdba func Sum= " + result2); //Print Sum: 6

    string ret2 = GetNameTypeResult("serial", 2, (a, b) =>
     {
         int sum = a + b + 1000;
         return sum.ToString();
     });

    Debug.WriteLine($"ret2= {ret2} with lambda");

    string ret2a = GetNameTypeResult("serial  2a test", 2, Sum);

    Debug.WriteLine($"ret2a= {ret2} with function Sum");

    string ret3 = GetNameTypeResult("serialWithout", 2, null);

    Debug.WriteLine($"ret3= {ret3} with null");

    GetNameTypeAdd("var2", 7, Add10);

    Debug.WriteLine($"only Add void - Action");
}

public string GetNameTypeResult(string variableName, int variableType, Func<int, int, string> result)
{
    string ret = "";

    string newResult = (result != null) ? result(5, 6) : "is null";

    ret = $"VariableName= {variableName}, variableType= {variableType}  result= {newResult}";

    return ret;
}

public string GetNameTypeAdd(string variableName, int variableType, Action<int> Add10)
{
    string ret = "";

    //(Add10 != null) ? Add10(5) : Add10(0);
    Add10(5);

    ret = $"VariableName= {variableName}, variableType= {variableType}";

    return ret;
}


public string Sum(int a, int b)
{
    string str = string.Format("Sum: {0}", (a + b));
    return str;
}

void Add10(int x)
{
    int increment = x + 10;
    Debug.WriteLine("increment = {0}", increment);
}
}