[DllImport ( "Shlwapi.dll", CharSet = CharSet.Auto )]
public static extern long StrFormatByteSize ( 
        long fileSize
        , [MarshalAs ( UnmanagedType.LPTStr )] StringBuilder buffer
        , int bufferSize );


/// <summary>
/// Converts a numeric value into a string that represents the number expressed as a size value in bytes, kilobytes, megabytes, or gigabytes, depending on the size.
/// </summary>
/// <param name="filelength">The numeric value to be converted.</param>
/// <returns>the converted string</returns>
public static string StrFormatByteSize (long filesize) {
     StringBuilder sb = new StringBuilder( 11 );
     StrFormatByteSize( filesize, sb, sb.Capacity );
     return sb.ToString();
}

来自:http://www.pinvoke.net/default.aspx/shlwapi/StrFormatByteSize.html

string[] suffixes = { "B", "KB", "MB", "GB", "TB", "PB", "EB", "ZB", "YB" };
int s = 0;
long size = fileInfo.Length;

while (size >= 1024)
{
    s++;
    size /= 1024;
}

string humanReadable = String.Format("{0} {1}", size, suffixes[s]);

1-liner(加上前缀常量)

const String prefixes = " KMGTPEY";
/// <summary> Returns the human-readable file size for an arbitrary, 64-bit file size. </summary>
public static String HumanSize(UInt64 bytes)
    => Enumerable
    .Range(0, prefixes.Length)
    .Where(i => bytes < 1024U<<(i*10))
    .Select(i => $"{(bytes>>(10*i-10))/1024:0.###} {prefixes[i]}B")
    .First();

或者，如果你想减少LINQ对象的分配，使用相同的for循环变量:

/// <summary>
/// Returns the human-readable file size for an arbitrary, 64-bit file size.
/// </summary>
public static String HumanSize(UInt64 bytes)
{
    const String prefixes = " KMGTPEY";
    for (var i = 0; i < prefixes.Length; i++)
        if (bytes < 1024U<<(i*10))
            return $"{(bytes>>(10*i-10))/1024:0.###} {prefixes[i]}B";

    throw new ArgumentOutOfRangeException(nameof(bytes));
}

如果你试图匹配Windows资源管理器的详细信息视图中显示的大小，这是你想要的代码:

[DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
private static extern long StrFormatKBSize(
    long qdw,
    [MarshalAs(UnmanagedType.LPTStr)] StringBuilder pszBuf,
    int cchBuf);

public static string BytesToString(long byteCount)
{
    var sb = new StringBuilder(32);
    StrFormatKBSize(byteCount, sb, sb.Capacity);
    return sb.ToString();
}

这不仅会与资源管理器完全匹配，而且还会为您提供翻译后的字符串，并匹配Windows版本的差异(例如在Win10中，K = 1000 vs.之前的版本K = 1024)。

int size = new FileInfo( filePath ).Length / 1024;
string humanKBSize = string.Format( "{0} KB", size );
string humanMBSize = string.Format( "{0} MB", size / 1024 );
string humanGBSize = string.Format( "{0} GB", size / 1024 / 1024 );

我的观点是:

千字节的前缀是kB(小写K) 由于这些函数用于表示目的，因此应该提供区域性，例如:CurrentCulture,“{0时。##}{1}"，文件大小，单位); 根据上下文，千字节可以是1000字节或1024字节。MB、GB等也是如此。