int num = random.Next(1000);

The following code returns a random number between min and max:

private int RandomNumber(int min, int max)
{
Random random = new Random();
return random.Next(min, max);
}

At some point, you may also want to generate random strings. I have created a method, which takes first parameter as the size of string and second parameter if you want the string to be lowercase.

/// <summary>
/// Generates a random string with the given length
/// </summary>
/// <param name="size">Size of the string</param>
/// <param name="lowerCase">If true, generate lowercase string</param>
/// <returns>Random string</returns>
private string RandomString(int size, bool lowerCase)
{
StringBuilder builder = new StringBuilder();
Random random = new Random();
char ch ;
for(int i=0; i<size; i++)
{
ch = Convert.ToChar(Convert.ToInt32(Math.Floor(26 * random.NextDouble() + 65))) ;
builder.Append(ch);
}
if(lowerCase)
return builder.ToString().ToLower();
return builder.ToString();
}

You can even combine the two methods - RandomNumber and RandomString to generate a combination of random string and numbers. For example, the following code generates a password of length 10 with first 4 letters lowercase, next 4 letters numbers, and last 2 letters as uppercase.

public string GetPassword()
{
StringBuilder builder = new StringBuilder();
builder.Append(RandomString(4, true));
builder.Append(RandomNumber(1000, 9999));
builder.Append(RandomString(2, false));
return builder.ToString();
}