Check whether the specified character has a surrogate code in C#

In C#, a surrogate is a character that represents a Unicode code point outside the Basic Multilingual Plane (BMP). Surrogate characters are used to encode Unicode characters that require more than 16 bits, using a pair of 16-bit values called high surrogate and low surrogate.

The Char.IsSurrogate() method checks whether a character at a specified position in a string is a surrogate character.

Syntax

Following is the syntax for checking surrogate characters −

bool result = Char.IsSurrogate(string, index);
bool result = Char.IsSurrogate(char);

Parameters

• string − The string containing the character to check

• index − The position of the character in the string

• char − The character to check directly

Return Value

Returns true if the character is a surrogate (either high or low surrogate), otherwise returns false.

Using Char.IsSurrogate() with String

Example

using System;

public class Demo {
    public static void Main() {
        string str = new String(new char[] { 'k', 'm', 'g', 't', '\uD800' });
        bool res = Char.IsSurrogate(str, 4);
        
        if (res)
            Console.WriteLine("Contains Surrogate value!");
        else
            Console.WriteLine("Does not contain Surrogate value!");
            
        // Check what type of surrogate it is
        if (Char.IsHighSurrogate(str, 4))
            Console.WriteLine("It's a high surrogate");
        else if (Char.IsLowSurrogate(str, 4))
            Console.WriteLine("It's a low surrogate");
    }
}

The output of the above code is −

Contains Surrogate value!
It's a high surrogate

Using Char.IsSurrogate() with Regular Characters

Example

using System;

public class Demo {
    public static void Main() {
        string str = new String(new char[] { 'k', 'm', 'g', 't', 'w' });
        bool res = Char.IsSurrogate(str, 4);
        
        if (res)
            Console.WriteLine("Contains Surrogate value!");
        else
            Console.WriteLine("Does not contain Surrogate value!");
            
        // Check individual character
        char ch = 'A';
        Console.WriteLine($"Character '{ch}' is surrogate: {Char.IsSurrogate(ch)}");
    }
}

The output of the above code is −

Does not contain Surrogate value!
Character 'A' is surrogate: False

Working with Surrogate Pairs

Example

using System;

public class Demo {
    public static void Main() {
        // Creating a string with emoji (requires surrogate pair)
        string emoji = "?"; // Smiling face emoji
        
        Console.WriteLine($"String: {emoji}");
        Console.WriteLine($"Length: {emoji.Length}"); // Will be 2, not 1
        
        // Check each character in the emoji
        for (int i = 0; i < emoji.Length; i++) {
            char ch = emoji[i];
            Console.WriteLine($"Position {i}: '{ch}' (U+{((int)ch):X4}) - IsSurrogate: {Char.IsSurrogate(ch)}");
            
            if (Char.IsHighSurrogate(ch))
                Console.WriteLine("  -> High surrogate");
            else if (Char.IsLowSurrogate(ch))
                Console.WriteLine("  -> Low surrogate");
        }
    }
}

The output of the above code is −

String: ?
Length: 2
Position 0: '?' (U+D83D) - IsSurrogate: True
  -> High surrogate
Position 1: '?' (U+DE00) - IsSurrogate: True
  -> Low surrogate

Conclusion

The Char.IsSurrogate() method is essential for working with Unicode characters beyond the Basic Multilingual Plane. It helps identify surrogate characters that are part of surrogate pairs used to represent emojis, mathematical symbols, and other extended Unicode characters in UTF-16 encoding.