About System.Web.Caching "Object references not set to an instance of an object" error

The initial contact with ASP. NET soon, want to make a cache mechanism on the backend, then use the System.Web.Caching cache class.

The logic to use the part is simple, but the total error when the value is taken

private static Cache cache = new cache ();

public static string Loadcache (String Key)
{
if (cache[key] = = NULL)//This step is an error
{
return null;
}
Else
{
Return Cache[key]. ToString ();
}
}

Search the web for a solution as follows:

Use "Cache cache = Httpruntime.cache;" Instead of "cache cache = new cache ();" To create an instance of the cache.

The test passed, but the reason is unclear, understand the great god please advise.

About System.Web.Caching "Object references not set to an instance of an object" error