Make first letter of a string upper case (with maximum performance)

Solution in different C# versions

C# 8 with at least .NET Core 3.0 or .NET Standard 2.1

public static class StringExtensions
{
    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))
        };
}

Since .NET Core 3.0 / .NET Standard 2.1 String.Concat() supports ReadonlySpan<char> which saves one allocation if we use .AsSpan(1) instead of .Substring(1).

C# 8

public static class StringExtensions
{
    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)
        };
}

C# 7

public static class StringExtensions
{
    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);
        }
    }
}

Really old answers

public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("ARGH!");
    return input.First().ToString().ToUpper() + String.Join("", input.Skip(1));
}

This version is shorter. For a faster solution, take a look at Diego's answer.

public static string FirstCharToUpper(string input)
{
    if (String.IsNullOrEmpty(input))
        throw new ArgumentException("ARGH!");
    return input.First().ToString().ToUpper() + input.Substring(1);
}

Probably the fastest solution is Darren's (There's even a benchmark) although I would change it's string.IsNullOrEmpty(s) validation to throw an exception since the original requirement expects for a first letter to exist so it can be uppercased. Note that this code works for a generic string and not particularly on valid values from the Textbox.

public string FirstLetterToUpper(string str)
{
    if (str == null)
        return null;

    if (str.Length > 1)
        return char.ToUpper(str[0]) + str.Substring(1);

    return str.ToUpper();
}

Old answer: This makes every first letter to upper case

public string ToTitleCase(string str)
{
    return CultureInfo.CurrentCulture.TextInfo.ToTitleCase(str.ToLower());
}

The right way is to use Culture:

System.Globalization.CultureInfo.CurrentCulture.TextInfo.ToTitleCase(word.ToLower())

Note: This will capitalise each word within a string, e.g. "red house" --> "Red House". The solution will also lower-case capitalisation within words, e.g. "old McDonald" --> "Old Mcdonald".

I took the fastest method from C# Uppercase First Letter - Dot Net Perls and converted to an extension method:

    /// <summary>
    /// Returns the input string with the first character converted to uppercase, or mutates any nulls passed into string.Empty
    /// </summary>
    public static string FirstLetterToUpperCaseOrConvertNullToEmptyString(this string s)
    {
        if (string.IsNullOrEmpty(s))
            return string.Empty;

        char[] a = s.ToCharArray();
        a[0] = char.ToUpper(a[0]);
        return new string(a);
    }

NOTE: The reason using ToCharArray is faster than the alternative char.ToUpper(s[0]) + s.Substring(1), is that only one string is allocated, whereas the Substring approach allocates a string for the substring, and then a second string to compose the final result.

Here is what this approach looks like, combined with the initial test from CarlosMuñoz's accepted answer:

    /// <summary>
    /// Returns the input string with the first character converted to uppercase
    /// </summary>
    public static string FirstLetterToUpperCase(this string s)
    {
        if (string.IsNullOrEmpty(s))
            throw new ArgumentException("There is no first letter");

        char[] a = s.ToCharArray();
        a[0] = char.ToUpper(a[0]);
        return new string(a);
    }