I suggest building a Dictionary<string, Operation> to map friendly names to enum constants and use normal naming conventions in the elements themselves.

enum Operation{ Equals, NotEquals, LessThan, GreaterThan };

var dict = new Dictionary<string, Operation> {
    { "Equals", Operation.Equals },
    { "Not Equals", Operation.NotEquals },
    { "Less Than", Operation.LessThan },
    { "Greater Than", Operation.GreaterThan }
};

var op = dict[str]; 

By the way, you can do (which I recommend against doing):

var op = (Operation)Enum.Parse(typeof(Operation), str.Replace(' ', '_'));

I suggest building a Dictionary<string, Operation> to map friendly names to enum constants and use normal naming conventions in the elements themselves.

enum Operation{ Equals, NotEquals, LessThan, GreaterThan };

var dict = new Dictionary<string, Operation> {
    { "Equals", Operation.Equals },
    { "Not Equals", Operation.NotEquals },
    { "Less Than", Operation.LessThan },
    { "Greater Than", Operation.GreaterThan }
};

var op = dict[str]; 

Alternatively, if you want to stick to your current method, you can do (which I recommend against doing):

var op = (Operation)Enum.Parse(typeof(Operation), str.Replace(' ', '_'));