When using the string.contains method , the comparison is done based on the exact string that is passed as parameter. In this case , the comparison is case-sensitive.

using System;
namespace ACConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = "This is a WeLcome string";
            var output = input.Contains("Welcome");
            Console.WriteLine(output);
            Console.ReadLine();
        }
    }
}

In the above code snippet , the result is false since the text Welcome is not found in the input string.

How to perform case insensitive string comparison for contains in C# ?

Below is a sample code snippet demonstrating how to do it.

using System;
using System.Globalization;
using System.Threading;

namespace ACConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = "This is a WeLcome string";
            var output = Thread.CurrentThread.CurrentCulture.CompareInfo.IndexOf(input, "Welcome", CompareOptions.IgnoreCase);
            Console.WriteLine(output);
            Console.ReadLine();
        }
    }
}