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);
        }
    }

}

最快的方法:

private string Capitalize(string s){
    if (string.IsNullOrEmpty(s))
    {
        return string.Empty;
    }
    char[] a = s.ToCharArray();
    a[0] = char.ToUpper(a[0]);
    return new string(a);
}

测试显示下一个结果(有1,000,000个符号作为输入的字符串):

测试结果

正如BobBeechey在回答这个问题时所建议的，下面的代码可以解决这个问题:

private void txt_fname_TextChanged(object sender, EventArgs e)
{
    char[] c = txt_fname.Text.ToCharArray();
    int j;
    for (j = 0; j < txt_fname.Text.Length; j++)
    {
        if (j==0) c[j]=c[j].ToString().ToUpper()[0];
        else c[j] = c[j].ToString().ToLower()[0];
    }
    txt_fname.Text = new string(c); 
    txt_fname.Select(txt_fname.Text.Length, 1);
}

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) : ""); }

我用这个来纠正名字。它的基本原理是，如果字符遵循特定的模式，则将其更改为大写。在这种情况下，我选择了空格和“Mc”的破折号。

private String CorrectName(String name)
{
    List<String> StringsToCapitalizeAfter = new List<String>() { " ", "-", "Mc" };
    StringBuilder NameBuilder = new StringBuilder();
    name.Select(c => c.ToString()).ToList().ForEach(c =>
    {
        c = c.ToLower();
        StringsToCapitalizeAfter.ForEach(s =>
        {
            if(String.IsNullOrEmpty(NameBuilder.ToString()) ||
               NameBuilder.ToString().EndsWith(s))
            {
                c = c.ToUpper();
            }
        });
        NameBuilder.Append(c);
    });
    return NameBuilder.ToString();
}

解决问题的可能方法:

   public static string FirstToUpper(this string lowerWord)
   {
       if (string.IsNullOrWhiteSpace(lowerWord) || string.IsNullOrEmpty(lowerWord))
            return lowerWord;
       return new StringBuilder(lowerWord.Substring(0, 1).ToUpper())
                 .Append(lowerWord.Substring(1))
                 .ToString();
   }