Question

In C# how we can use SHA1 automatically?
Is SHA1 better than MD5?(We use hashing for user name and password and need speed for authentication)

Answer 1

Not sure what you mean by automatically, but you should really use SHA256 and higher. Also always use a Salt (code) with your hashes. A side note, after time has passed, using hardened hashes is far better than using a plain speed-based hashing function. I.e.: hashing over a few hundred iterations, or using already proven hashing functions such as bcrypt (which is mentioned below I believe). A code sample for using a SHA256 hash function in .NET is as follows:

byte[] data = new byte[DATA_SIZE];
byte[] result;

using(SHA256 shaM = new SHA256Managed()) {
    result = shaM.ComputeHash(data);
}

Will do the trick for you using SHA256 and is found at MSDN.

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Sidenote on the “cracking” of SHA1: Putting the cracking of SHA-1 in perspective