您正在该方法中创建Random实例，这将导致它在快速连续调用时返回相同的值。我会这样做:

private static Random random = new Random((int)DateTime.Now.Ticks);//thanks to McAden
private string RandomString(int size)
    {
        StringBuilder builder = new StringBuilder();
        char ch;
        for (int i = 0; i < size; i++)
        {
            ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65)));                 
            builder.Append(ch);
        }

        return builder.ToString();
    }

// get 1st random string 
string Rand1 = RandomString(4);

// get 2nd random string 
string Rand2 = RandomString(4);

// creat full rand string
string docNum = Rand1 + "-" + Rand2;

(修改后的代码版本)

一个LINQ单行程序(假设私有静态随机随机)…

public static string RandomString(int length)
{
    return new string(Enumerable.Range(0, length).Select(_ => (char)Random.Next('a', 'z')).ToArray());
}

您应该在构造函数中初始化一个类级随机对象，并在每次调用时重用它(这延续了相同的伪随机数序列)。无参数构造函数已经用Environment为生成器播下种子。TickCount内部。

我的RandomString()方法来生成一个随机字符串。

private static readonly Random _rand = new Random();

/// <summary>
/// Generate a random string.
/// </summary>
/// <param name="length">The length of random string. The minimum length is 3.</param>
/// <returns>The random string.</returns>
public string RandomString(int length)
{
    length = Math.Max(length, 3);

    byte[] bytes = new byte[length];
    _rand.NextBytes(bytes);
    return Convert.ToBase64String(bytes).Substring(0, length);
}

这是因为Random的每个新实例都在快速调用中生成相同的数字。不要一直创建新实例，只需调用next()并在方法之外声明随机类。

这是我的解决方案:

private string RandomString(int length)
{
    char[] symbols = { 
                            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
                            'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
                            'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'                             
                        };

    Stack<byte> bytes = new Stack<byte>();
    string output = string.Empty;

    for (int i = 0; i < length; i++)
    {
        if (bytes.Count == 0)
        {
            bytes = new Stack<byte>(Guid.NewGuid().ToByteArray());
        }
        byte pop = bytes.Pop();
        output += symbols[(int)pop % symbols.Length];
    }
    return output;
}

// get 1st random string 
string Rand1 = RandomString(4);

// get 2nd random string 
string Rand2 = RandomString(4);

// create full rand string
string docNum = Rand1 + "-" + Rand2;