string.Replace() / Contains()

string.Replace(string oldValue, string newValue)

// Returns true if the string contains value.
string.Contains(string value)

// Returns true if the string starts with value.
string.StartsWith(string value)

// Returns true if the string ends with value.
string.EndsWith(string value)

Program.cs

using System;

string sentence = "C# is an object-oriented programming language. C# was developed by Microsoft.";

// Use Replace() to substitute all occurrences
string replaced = sentence.Replace("C#", "C-Sharp");
Console.WriteLine(replaced);
// C-Sharp is an object-oriented programming language. C-Sharp was developed by Microsoft.

// Use Contains() to check for a substring
Console.WriteLine(sentence.Contains("Microsoft")); // True
Console.WriteLine(sentence.Contains("Java")); // False

// Use StartsWith() / EndsWith() to check the beginning and end of a string
string url = "https://wp-p.info";
Console.WriteLine(url.StartsWith("https")); // True
Console.WriteLine(url.EndsWith(".info")); // True
Console.WriteLine(url.EndsWith(".com")); // False

This produces the following output:

dotnet run
C-Sharp is an object-oriented programming language. C-Sharp was developed by Microsoft.
True
False
True
True
False

By default, these methods are case-sensitive. To compare without case sensitivity, pass StringComparison.OrdinalIgnoreCase.

CaseInsensitive.cs

using System;

string text = "Hello, C# World!";

// Case-sensitive (default)
Console.WriteLine(text.Contains("hello")); // False
Console.WriteLine(text.StartsWith("hello")); // False

// Case-insensitive
Console.WriteLine(text.Contains("hello", StringComparison.OrdinalIgnoreCase)); // True
Console.WriteLine(text.StartsWith("hello", StringComparison.OrdinalIgnoreCase)); // True

// Use Replace() to remove characters by replacing them with an empty string
string noSpaces = "Hello, C# World!".Replace(" ", "");
Console.WriteLine(noSpaces); // Hello,C#World!

This produces the following output:

dotnet run
False
False
True
True
Hello,C#World!

An example combining StartsWith() and Contains() for URL validation.

UrlValidation.cs

using System;

string[] urls = {
    "https://wp-p.info",
    "http://example.com",
    "ftp://files.example.org",
    "https://api.example.jp/data"
};

foreach (string url in urls) {
    bool isHttps = url.StartsWith("https://");
    bool isJpDomain = url.Contains(".jp");
    Console.WriteLine($"{url} → HTTPS: {isHttps}, .jp domain: {isJpDomain}");
}

This produces the following output:

dotnet run
https://wp-p.info → HTTPS: True, .jp domain: False
http://example.com → HTTPS: False, .jp domain: False
ftp://files.example.org → HTTPS: False, .jp domain: False
https://api.example.jp/data → HTTPS: True, .jp domain: True

Common Mistake: Forgetting to Use the Return Value of Replace()

C# strings are immutable. Replace() does not modify the original string — it returns a new one. If you do not assign the return value, the replacement result is lost.

using System;

// NG: The return value of Replace() is not assigned to a variable
string name = "Yagami Iori";
name.Replace("Iori", "Kyo"); // The new string is discarded
Console.WriteLine(name); // Yagami Iori (unchanged)

This produces the following output:

dotnet run
Yagami Iori

using System;

// OK: Assign the return value to a variable
string name = "Yagami Iori";
string replaced = name.Replace("Iori", "Kyo");
Console.WriteLine(replaced); // Yagami Kyo

This produces the following output:

dotnet run
Yagami Kyo

All of these methods are case-sensitive. To perform a case-insensitive comparison, pass StringComparison.OrdinalIgnoreCase as an argument, or normalize the string first using ToUpper() / ToLower().

Because C# strings are immutable, Replace() does not modify the original string — it returns a new one. Make sure to assign the return value to a variable.

To split or join strings, see Split() / string.Join().