You haven’t quite mentioned the context of your question, for making a guess here.

Easiest way to check if “not” contains would be to use negation operator !. For example,

To check to ensure does not contain a particular substring
if(!str.Contains(substring))
To ensure a particular item is not part of collection

if(!list.Contains(item))

I find the question pretty confusing if not silly.

1. In plain C# there is no such thing as “the opposite of ‘Contains’ in C#”
2. In LINQ you have the Method “Exclude”. LINQ is related to C#, as it is in the BCL starting with version 3.5

you can use not operator for check enter value is not in list .

if (!myList.Contains("name"))
{
    myList.Add("name");
}

or you can use any method to perform same.

if (!myList.Any(s => s == "name"))
{
    myList.Add("name");
}

Use ! operator