Helper and utility classes are often designed to never be instantiated. In C# 1.0 developers had to enforce this manually by making the constructor private and sealing the class, but nothing prevented the class from accidentally containing instance members.

C# 2.0 adds the static modifier to classes. A static class cannot be instantiated or inherited, and the compiler enforces that it contains only static members.

Code

Now

static class MathHelper
{
    public static double DegreesToRadians(double degrees)
    {
        return degrees * Math.PI / 180.0;
    }
}

Before

sealed class MathHelper
{
    private MathHelper() { }

    public static double DegreesToRadians(double degrees)
    {
        return degrees * Math.PI / 180.0;
    }
}

Notes

• The compiler will emit an error if you try to add instance members, inherit from, or instantiate a static class
• Static classes are also required for declaring Extension methods ^{C# 3.0}

More information

• C# Language Reference