下面的函数对所有方式都是正确的:

static string UppercaseWords(string value)
{
    char[] array = value.ToCharArray();
    // Handle the first letter in the string.
    if (array.Length >= 1)
    {
        if (char.IsLower(array[0]))
        {
            array[0] = char.ToUpper(array[0]);
        }
    }
    // Scan through the letters, checking for spaces.
    // ... Uppercase the lowercase letters following spaces.
    for (int i = 1; i < array.Length; i++)
    {
        if (array[i - 1] == ' ')
        {
            if (char.IsLower(array[i]))
            {
                array[i] = char.ToUpper(array[i]);
            }
        }
    }
    return new string(array);
}

我在这里找到的。

我从c#大写首字母- Dot Net Perls中获得了最快的方法，并将其转换为扩展方法:

    /// <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);
    }

注意:使用ToCharArray比替代char.ToUpper(s[0]) + s.Substring(1)更快的原因是只分配了一个字符串，而Substring方法为子字符串分配了一个字符串，然后再分配一个字符串来组成最终结果。

下面是这种方法的样子，结合CarlosMuñoz接受的答案的初始测试:

    /// <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);
    }

这也可以使用Take, Skip和Aggregate:

    public static string FirstCharToUpper(this string text) 
    {
      if (String.IsNullOrEmpty(text)) return String.Empty;
      var first = text.Take(1).ToArray()[0].ToString().ToUpper();
      var rest = text.Skip(1).Aggregate("", ((xs, x) => xs + x));
      return first + rest;
    }

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();
}

旧的回答: 这使得每个首字母都是大写的

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

由于这个问题是关于最大化性能的，我采用了Darren的版本来使用跨度，这减少了垃圾，并提高了大约10%的速度。

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

    Span<char> a = stackalloc char[s.Length];
    s.AsSpan(1).CopyTo(a.Slice(1));
    a[0] = char.ToUpper(s[0]);
    return new string(a);
}

性能

完整的测试代码

using System;
using System.Linq;

using BenchmarkDotNet.Attributes;

namespace CorePerformanceTest
{
    public class StringUpperTest
    {
        [Params("red", "red house")]
        public string Data;

        [Benchmark]
        public string Carlos() => Data.Carlos();

        [Benchmark]
        public string Darren() => Data.Darren();

        [Benchmark]
        public string Marcell() => Data.Marcell();
    }

    internal static class StringExtensions
    {
        public static string Carlos(this string input) =>
            input switch
            {
                null => throw new ArgumentNullException(nameof(input)),
                "" => throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input)),
                _ => input.First().ToString().ToUpper() + input.Substring(1)
            };

        public static string Darren(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);
        }

        public static string Marcell(this string s)
        {
            if (string.IsNullOrEmpty(s))
                throw new ArgumentException("There is no first letter");

            Span<char> a = stackalloc char[s.Length];
            s.AsSpan(1).CopyTo(a.Slice(1));
            a[0] = char.ToUpper(s[0]);
            return new string(a);
        }
    }

}

第一个字母和空格后面的每个字母都大写，其他字母都小写。

public string CapitalizeFirstLetterAfterSpace(string input)
{
    System.Text.StringBuilder sb = new System.Text.StringBuilder(input);
    bool capitalizeNextLetter = true;
    for(int pos = 0; pos < sb.Length; pos++)
    {
        if(capitalizeNextLetter)
        {
            sb[pos]=System.Char.ToUpper(sb[pos]);
            capitalizeNextLetter = false;
        }
        else
        {
            sb[pos]=System.Char.ToLower(sb[pos]);
        }

        if(sb[pos]=' ')
        {
            capitalizeNextLetter=true;
        }
    }
}