Is C# a type safe language? How about my example

Question

I am testing the following code in C# and it can be run successfully. My question is I can assign one type of data to another type of data in the following example, but why it is still called type-safe language? Thanks.

using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text.RegularExpressions;

    namespace Rextester
    {
        public class Program
        {
            public static void Main(string[] args)
            {
               var intNum = 5;
               var strNum = "5";
               var result = intNum + strNum;         
               Console.WriteLine(result);
            }
        }
    }

It can be compiled successfully and result is 55.

Answer 1

Yes it is. Your example uses var which has a very different meaning in C# than it has say, in JavaScript. In C# it is more of a syntatic sugar. The code you wrote is equivalent of the following -

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
           int intNum = 5;
           string strNum = "5";
           string result = String.Concat(intNum, strNum);         
           Console.WriteLine(result);
        }
    }
}

So basically, the compiler looks at the right side of a var declaration to decide on the right type. It's sort of telling the compiler to get the type (strictly compile time) because I am too lazy to bother with that.

But it serves a larger purpose, it lets you deal with anonymous types.

Finally, to demonstrate beyond any doubt that var is truly type safe, try the following code snippet...

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
           var intNum = 5;
           var strNum = "5";
           var result = intNum + strNum;
           // Let's re-purpose result to store an int
           result = 6;
           // or this
           result = intNum;
           Console.WriteLine(result);
        }
    }
}

This is perfectly valid in a type agnostic language (again say JavaScript).