我想提供一个“最大性能”的答案。在我看来，“最大性能”的答案抓住了所有的场景，并提供了解释这些场景的问题的答案。所以，这是我的答案。原因如下:

IsNullOrWhiteSpace accounts for strings that are just spaces or null/empty. .Trim() removes white space from the front and back of the string. .First() takes the first element of an IEnumerable<TSource> (or string). We should check to see if it is a letter that can/should be uppercase. We then add the rest of the string, only if the length indicates we should. By .NET best practice, we should provide a culture under System.Globalization.CultureInfo. Providing them as optional parameters makes this method totally reusable, without having to type the chosen culture every time. I also noticed that my and most of these answers did not maintain the whitespace at the beginning of the string. This will also show how to maintain that whitespace. //Capitalize the first letter disregard all chars using regex. public static string RegCapString(this string instring, string culture = "en-US", bool useSystem = false) { if (string.IsNullOrWhiteSpace(instring)) { return instring; } var m = Regex.Match(instring, "[A-Za-z]").Index; return instring.Substring(0, m) + instring[m].ToString().ToUpper(new CultureInfo(culture, useSystem)) + instring.Substring(m + 1); } //Capitalize first char if it is a letter disregard white space. public static string CapString(this string instring, string culture = "en-US", bool useSystem = false) { if (string.IsNullOrWhiteSpace(instring) || !char.IsLetter(instring.Trim().First())) { return instring; } var whiteSpaces = instring.Length - instring.TrimStart().Length; return (new string(' ', whiteSpaces)) + instring.Trim().First().ToString().ToUpper(new CultureInfo(culture, useSystem)) + ((instring.TrimStart().Length > 1) ? instring.Substring(whiteSpaces + 1) : ""); }

 private string capitalizeFirstCharacter(string format)
 {
     if (string.IsNullOrEmpty(format))
         return string.Empty;
     else
         return char.ToUpper(format[0]) + format.ToLower().Substring(1);
 }

不同c#版本的解决方案

c# 8，至少。net Core 3.0或。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))
        };
}

自从。net Core 3.0 / . net Standard 2.1以来，String.Concat()支持ReadonlySpan<char>，如果我们使用. asspan(1)而不是. substring(1)，就节省了一个分配。

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);
        }
    }
}

非常古老的答案

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

这个版本比较短。为了更快地解决问题，请看迭戈的回答。

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

可能最快的解决方案是Darren的(甚至有一个基准测试)，尽管我会改变它的string.IsNullOrEmpty(s)验证来抛出异常，因为最初的需求期望第一个字母存在，以便它可以大写。注意，这段代码适用于一般字符串，而不是文本框中的有效值。

下面是对我有用的代码:

private string StringLetterUppercase(string input)
{
    if (input == null)
    {
        throw new ArgumentNullException(nameof(input));
    }
    else if (input == "")
    {
        throw new ArgumentException($"{nameof(input)} cannot be empty", nameof(input));
    }
    else
    {
        return input.First().ToString().ToUpper() + input.Substring(1);
    }
}

扩展Carlos上面的问题，如果你想要大写多个句子，你可以使用下面的代码:

    /// <summary>
    /// Capitalize first letter of every sentence. 
    /// </summary>
    /// <param name="inputSting"></param>
    /// <returns></returns>
    public string CapitalizeSentences (string inputSting)
    {
        string result = string.Empty;
        if (!string.IsNullOrEmpty(inputSting))
        {
            string[] sentences = inputSting.Split('.');

            foreach (string sentence in sentences)
            {
                result += string.Format ("{0}{1}.", sentence.First().ToString().ToUpper(), sentence.Substring(1)); 
            }
        }

        return result; 
    }

如果你只关心第一个字母是否大写，而不关心字符串的其他部分，你可以只选择第一个字符，使其大写，并将其与没有原始第一个字符的字符串的其余部分连接起来。

String word ="red house";
word = word[0].ToString().ToUpper() + word.Substring(1, word.length -1);
//result: word = "Red house"

我们需要将第一个字符转换为ToString()，因为我们将其作为Char数组读取，而Char类型没有ToUpper()方法。