Code
  string md5Hash = string.Empty;
 FileStream stream = File.Open(filename, FileMode.Open, FileAccess.Read);
                try
                {                  System.Security.Cryptography.MD5CryptoServiceProvider md5 = new System.Security.Cryptography.MD5CryptoServiceProvider();
                    byte[] b = md5.ComputeHash(stream);
                    md5Hash = System.BitConverter.ToString(b).Replace("-", "");
                }
                catch
                {
                    stream.Close();
                    continue;
                }
                stream.Close();

Code
using System; 
using System.Collections; 
using System.IO;

public class MD5 {
    //static state variables 
    private static UInt32 A;
    private static UInt32 B;
    private static UInt32 C;
    private static UInt32 D;

    //number of bits to rotate in tranforming 
    private const int S11 = 7;
    private const int S12 = 12;
    private const int S13 = 17;
    private const int S14 = 22;
    private const int S21 = 5;
    private const int S22 = 9;
    private const int S23 = 14;
    private const int S24 = 20;
    private const int S31 = 4;
    private const int S32 = 11;
    private const int S33 = 16;
    private const int S34 = 23;
    private const int S41 = 6;
    private const int S42 = 10;
    private const int S43 = 15;
    private const int S44 = 21;


    /* F, G, H and I are basic MD5 functions. 
    * 四个非线性函数: 
    * 
    * F(X,Y,Z) =(X&Y)|((~X)&Z) 
    * G(X,Y,Z) =(X&Z)|(Y&(~Z)) 
    * H(X,Y,Z) =X^Y^Z 
    * I(X,Y,Z)=Y^(X|(~Z)) 
    * 
    * （&与，|或，~非，^异或） 
    */
    private static UInt32 F(UInt32 x, UInt32 y, UInt32 z) {
        return (x & y) | ((~x) & z);
    }
    private static UInt32 G(UInt32 x, UInt32 y, UInt32 z) {
        return (x & z) | (y & (~z));
    }
    private static UInt32 H(UInt32 x, UInt32 y, UInt32 z) {
        return x ^ y ^ z;
    }
    private static UInt32 I(UInt32 x, UInt32 y, UInt32 z) {
        return y ^ (x | (~z));
    }

    /* FF, GG, HH, and II transformations for rounds 1, 2, 3, and 4. 
    * Rotation is separate from addition to prevent recomputation. 
    */
    private static void FF(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti) {
        a = a + F(b, c, d) + mj + ti;
        a = a << s | a >> (32 - s);
        a += b;
    }
    private static void GG(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti) {
        a = a + G(b, c, d) + mj + ti;
        a = a << s | a >> (32 - s);
        a += b;
    }
    private static void HH(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti) {
        a = a + H(b, c, d) + mj + ti;
        a = a << s | a >> (32 - s);
        a += b;
    }
    private static void II(ref UInt32 a, UInt32 b, UInt32 c, UInt32 d, UInt32 mj, int s, UInt32 ti) {
        a = a + I(b, c, d) + mj + ti;
        a = a << s | a >> (32 - s);
        a += b;
    }

    private static void MD5_Init() {
        A = 0x67452301; //in memory, this is 0x01234567 
        B = 0xefcdab89; //in memory, this is 0x89abcdef 
        C = 0x98badcfe; //in memory, this is 0xfedcba98 
        D = 0x10325476; //in memory, this is 0x76543210 
    }

    private static UInt32[] MD5_Append(byte[] input) {
        int zeros = 0;
        int ones = 1;
        int size = 0;
        int n = input.Length;
        int m = n % 64;
        if (m < 56) {
            zeros = 55 - m;
            size = n - m + 64;
        }
        else if (m == 56) {
            zeros = 0;
            ones = 0;
            size = n + 8;
        }
        else {
            zeros = 63 - m + 56;
            size = n + 64 - m + 64;
        }

        ArrayList bs = new ArrayList(input);
        if (ones == 1) {
            bs.Add((byte)0x80); // 0x80 = $10000000 
        }
        for (int i = 0; i < zeros; i++) {
            bs.Add((byte)0);
        }

        UInt64 N = (UInt64)n * 8;
        byte h1 = (byte)(N & 0xFF);
        byte h2 = (byte)((N >> 8) & 0xFF);
        byte h3 = (byte)((N >> 16) & 0xFF);
        byte h4 = (byte)((N >> 24) & 0xFF);
        byte h5 = (byte)((N >> 32) & 0xFF);
        byte h6 = (byte)((N >> 40) & 0xFF);
        byte h7 = (byte)((N >> 48) & 0xFF);
        byte h8 = (byte)(N >> 56);
        bs.Add(h1);
        bs.Add(h2);
        bs.Add(h3);
        bs.Add(h4);
        bs.Add(h5);
        bs.Add(h6);
        bs.Add(h7);
        bs.Add(h8);
        byte[] ts = (byte[])bs.ToArray(typeof(byte));

        /* Decodes input (byte[]) into output (UInt32[]). Assumes len is 
        * a multiple of 4. 
        */
        UInt32[] output = new UInt32[size / 4];
        for (Int64 i = 0, j = 0; i < size; j++, i += 4) {
            output[j] = (UInt32)(ts[i] | ts[i + 1] << 8 | ts[i + 2] << 16 | ts[i + 3] << 24);
        }
        return output;
    }
    private static UInt32[] MD5_Trasform(UInt32[] x) {

        UInt32 a, b, c, d;

        for (int k = 0; k < x.Length; k += 16) {
            a = A;
            b = B;

            c = C;
            d = D;

            /* Round 1 */
            FF(ref a, b, c, d, x[k + 0], S11, 0xd76aa478); /* 1 */
            FF(ref d, a, b, c, x[k + 1], S12, 0xe8c7b756); /* 2 */
            FF(ref c, d, a, b, x[k + 2], S13, 0x242070db); /* 3 */
            FF(ref b, c, d, a, x[k + 3], S14, 0xc1bdceee); /* 4 */
            FF(ref a, b, c, d, x[k + 4], S11, 0xf57c0faf); /* 5 */
            FF(ref d, a, b, c, x[k + 5], S12, 0x4787c62a); /* 6 */
            FF(ref c, d, a, b, x[k + 6], S13, 0xa8304613); /* 7 */
            FF(ref b, c, d, a, x[k + 7], S14, 0xfd469501); /* 8 */
            FF(ref a, b, c, d, x[k + 8], S11, 0x698098d8); /* 9 */
            FF(ref d, a, b, c, x[k + 9], S12, 0x8b44f7af); /* 10 */
            FF(ref c, d, a, b, x[k + 10], S13, 0xffff5bb1); /* 11 */
            FF(ref b, c, d, a, x[k + 11], S14, 0x895cd7be); /* 12 */
            FF(ref a, b, c, d, x[k + 12], S11, 0x6b901122); /* 13 */
            FF(ref d, a, b, c, x[k + 13], S12, 0xfd987193); /* 14 */
            FF(ref c, d, a, b, x[k + 14], S13, 0xa679438e); /* 15 */
            FF(ref b, c, d, a, x[k + 15], S14, 0x49b40821); /* 16 */

            /* Round 2 */
            GG(ref a, b, c, d, x[k + 1], S21, 0xf61e2562); /* 17 */
            GG(ref d, a, b, c, x[k + 6], S22, 0xc040b340); /* 18 */
            GG(ref c, d, a, b, x[k + 11], S23, 0x265e5a51); /* 19 */
            GG(ref b, c, d, a, x[k + 0], S24, 0xe9b6c7aa); /* 20 */
            GG(ref a, b, c, d, x[k + 5], S21, 0xd62f105d); /* 21 */
            GG(ref d, a, b, c, x[k + 10], S22, 0x2441453); /* 22 */
            GG(ref c, d, a, b, x[k + 15], S23, 0xd8a1e681); /* 23 */
            GG(ref b, c, d, a, x[k + 4], S24, 0xe7d3fbc8); /* 24 */
            GG(ref a, b, c, d, x[k + 9], S21, 0x21e1cde6); /* 25 */
            GG(ref d, a, b, c, x[k + 14], S22, 0xc33707d6); /* 26 */
            GG(ref c, d, a, b, x[k + 3], S23, 0xf4d50d87); /* 27 */
            GG(ref b, c, d, a, x[k + 8], S24, 0x455a14ed); /* 28 */
            GG(ref a, b, c, d, x[k + 13], S21, 0xa9e3e905); /* 29 */
            GG(ref d, a, b, c, x[k + 2], S22, 0xfcefa3f8); /* 30 */
            GG(ref c, d, a, b, x[k + 7], S23, 0x676f02d9); /* 31 */
            GG(ref b, c, d, a, x[k + 12], S24, 0x8d2a4c8a); /* 32 */

            /* Round 3 */
            HH(ref a, b, c, d, x[k + 5], S31, 0xfffa3942); /* 33 */
            HH(ref d, a, b, c, x[k + 8], S32, 0x8771f681); /* 34 */
            HH(ref c, d, a, b, x[k + 11], S33, 0x6d9d6122); /* 35 */
            HH(ref b, c, d, a, x[k + 14], S34, 0xfde5380c); /* 36 */
            HH(ref a, b, c, d, x[k + 1], S31, 0xa4beea44); /* 37 */
            HH(ref d, a, b, c, x[k + 4], S32, 0x4bdecfa9); /* 38 */
            HH(ref c, d, a, b, x[k + 7], S33, 0xf6bb4b60); /* 39 */
            HH(ref b, c, d, a, x[k + 10], S34, 0xbebfbc70); /* 40 */
            HH(ref a, b, c, d, x[k + 13], S31, 0x289b7ec6); /* 41 */
            HH(ref d, a, b, c, x[k + 0], S32, 0xeaa127fa); /* 42 */
            HH(ref c, d, a, b, x[k + 3], S33, 0xd4ef3085); /* 43 */
            HH(ref b, c, d, a, x[k + 6], S34, 0x4881d05); /* 44 */
            HH(ref a, b, c, d, x[k + 9], S31, 0xd9d4d039); /* 45 */
            HH(ref d, a, b, c, x[k + 12], S32, 0xe6db99e5); /* 46 */
            HH(ref c, d, a, b, x[k + 15], S33, 0x1fa27cf8); /* 47 */
            HH(ref b, c, d, a, x[k + 2], S34, 0xc4ac5665); /* 48 */

            /* Round 4 */
            II(ref a, b, c, d, x[k + 0], S41, 0xf4292244); /* 49 */
            II(ref d, a, b, c, x[k + 7], S42, 0x432aff97); /* 50 */
            II(ref c, d, a, b, x[k + 14], S43, 0xab9423a7); /* 51 */
            II(ref b, c, d, a, x[k + 5], S44, 0xfc93a039); /* 52 */
            II(ref a, b, c, d, x[k + 12], S41, 0x655b59c3); /* 53 */
            II(ref d, a, b, c, x[k + 3], S42, 0x8f0ccc92); /* 54 */
            II(ref c, d, a, b, x[k + 10], S43, 0xffeff47d); /* 55 */
            II(ref b, c, d, a, x[k + 1], S44, 0x85845dd1); /* 56 */
            II(ref a, b, c, d, x[k + 8], S41, 0x6fa87e4f); /* 57 */
            II(ref d, a, b, c, x[k + 15], S42, 0xfe2ce6e0); /* 58 */
            II(ref c, d, a, b, x[k + 6], S43, 0xa3014314); /* 59 */
            II(ref b, c, d, a, x[k + 13], S44, 0x4e0811a1); /* 60 */
            II(ref a, b, c, d, x[k + 4], S41, 0xf7537e82); /* 61 */
            II(ref d, a, b, c, x[k + 11], S42, 0xbd3af235); /* 62 */
            II(ref c, d, a, b, x[k + 2], S43, 0x2ad7d2bb); /* 63 */
            II(ref b, c, d, a, x[k + 9], S44, 0xeb86d391); /* 64 */

            A += a;
            B += b;
            C += c;
            D += d;
        }
        return new UInt32[] { A, B, C, D };
    }
    public static byte[] MD5Array(byte[] input) {
        MD5_Init();
        UInt32[] block = MD5_Append(input);
        UInt32[] bits = MD5_Trasform(block);

        /* Encodes bits (UInt32[]) into output (byte[]). Assumes len is 
        * a multiple of 4. 
        */
        byte[] output = new byte[bits.Length * 4];
        for (int i = 0, j = 0; i < bits.Length; i++, j += 4) {
            output[j] = (byte)(bits[i] & 0xff);
            output[j + 1] = (byte)((bits[i] >> 8) & 0xff);
            output[j + 2] = (byte)((bits[i] >> 16) & 0xff);
            output[j + 3] = (byte)((bits[i] >> 24) & 0xff);
        }
        return output;
    }

    public static string ArrayToHexString(byte[] array, bool uppercase) {
        string hexString = "";
        string format = "x2";
        if (uppercase) {
            format = "X2";
        }
        foreach (byte b in array) {
            hexString += b.ToString(format);
        }
        return hexString;
    }

    public static string MDString(string message) {
        char[] c = message.ToCharArray();
        byte[] b = new byte[c.Length];
        for (int i = 0; i < c.Length; i++) {
            b[i] = (byte)c[i];
        }
        byte[] digest = MD5Array(b);
        return ArrayToHexString(digest, false);
    }
    public static string MDFile(string fileName) {
        FileStream fs = File.Open(fileName, FileMode.Open, FileAccess.Read);
        byte[] array = new byte[fs.Length];
        fs.Read(array, 0, (int)fs.Length);
        byte[] digest = MD5Array(array);
        fs.Close();
        return ArrayToHexString(digest, false);
    }
}