How Can I Capitalize the First Letter of a String in C# with Optimal Performance?

Efficiently capitalize the first letter of C# string

Introduction

Capitalizing the first letter of a string is a common task in many programming applications. In order to optimize performance, it is crucial to choose an efficient method. This article explores several ways to implement capitalization in C#, and focuses on analyzing their performance differences.

Code Example

C# 8, .NET Core 3.0 or .NET Standard 2.1

<code class="language-csharp">public static string FirstCharToUpper(this string input) =>
    input switch
    {
        null => throw new ArgumentNullException(nameof(input)),
        "" => throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input)),
        _ => string.Concat(input[0].ToString().ToUpper(), input.AsSpan(1))
    };</code>

C# 8

<code class="language-csharp">public static string FirstCharToUpper(this string input) =>
    input switch
    {
        null => throw new ArgumentNullException(nameof(input)),
        "" => throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input)),
        _ => input[0].ToString().ToUpper() + input.Substring(1)
    };</code>

C# 7

<code class="language-csharp">public static string FirstCharToUpper(this string input)
{
    switch (input)
    {
        case null: throw new ArgumentNullException(nameof(input));
        case "": throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input));
        default: return input[0].ToString().ToUpper() + input.Substring(1);
    }
}</code>

Old version (not recommended, poor performance)

<code class="language-csharp">public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("ARGH!");
    return input.First().ToString().ToUpper() + String.Join("", input.Skip(1));
}</code>

<code class="language-csharp">public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("ARGH!");
    return input.First().ToString().ToUpper() + input.Substring(1);
}</code>

Performance Considerations

Among these code snippets, the method with the least memory allocation and the most efficient string manipulation has the best performance. The first solution uses ReadonlySpan<char> from .NET Core 3.0 or .NET Standard 2.1, which provides superior performance compared to other methods.

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