c# - StartsWith 应该只用于订购吗？

Question

我正在阅读 Microsoft 的 Best Practices for Using Strings in the .NET Framework .

它给出了以下示例作为对 StringComparison.OrdinalIgnoreCase 的介绍:

public static bool IsFileURI(string path) 
{
    return path.StartsWith("FILE:", StringComparison.OrdinalIgnoreCase);
}

到目前为止，还不错。但它接着说:

However, the preceding example uses the String.StartsWith(String, StringComparison) method to test for equality. Because the purpose of the comparison is to test for equality instead of ordering the strings, a better alternative is to call the Equals method, as shown in the following example.

public static bool IsFileURI(string path)
{
    if (path.Length < 5) return false;

    return String.Equals(path.Substring(0, 5), "FILE:", 
                         StringComparison.OrdinalIgnoreCase);
}   

我正在努力了解为什么第二个版本更好。我可以理解从 CompareTo(比较)切换到 Equals(相等)，但是 StartsWith 不也是一个相等测试吗？我是否遗漏了什么或者这是一个文档错误？

Answer 1

不是真正的答案，但 StartsWith() 是一个相等性测试，我认为这是一个文档错误，但我很想知道性能，所以我做了使用以下代码的基准:

class Program {
    static void Main( string[ ] args ) {
        Stopwatch sw = Stopwatch.StartNew( );
        for ( int i = 0; i < 1000000000; i++ ) //1 billion times
            IsFileURI1( "File:\\ThisIsATest" );
        sw.Stop( );
        Console.WriteLine( "String.StartsWith(): " + sw.ElapsedMilliseconds.ToString( ) );

        sw.Restart( );
        for ( int i = 0; i < 1000000000; i++ ) //1 billion times
            IsFileURI2( "File:\\ThisIsATest" );
        sw.Stop( );
        Console.WriteLine( "String.Equals(): " + sw.ElapsedMilliseconds.ToString( ) );
    }

    public static bool IsFileURI1( string path ) {
        return path.StartsWith( "FILE:", StringComparison.OrdinalIgnoreCase );
    }

    public static bool IsFileURI2( string path ) {
        if ( path.Length < 5 ) return false;

        return String.Equals( path.Substring( 0, 5 ), "FILE:", StringComparison.OrdinalIgnoreCase );
    }   
}

结果是(毫秒):

String.StartsWith(): 90102 
String.Equals(): 73113

因此，就性能而言，第二种解决方案更好，而且快 20%。