Terniary operator in linq

That doesn't make any sense.

The code you posted does this:

if Model does not equal ""
    // This will return a boolean value.
    return (x.ModelNumber.ToLower().Contains(Model.ToLower())
else
    // This will return whatever ModelNumber is.
    return x.ModelNumber

I highly doubt your ModelNumber is a boolean so your two return types are different. In a Ternary operation, that is a recipe for disaster.

From your code I am guessing that you are trying to filter and modify your code at the same time, which usually is not possible to do out of the box.

If I understood you snippet correctly you want to return a "ModelNumber" property if "Model" is not empty and it is contained it currently iterated element.

Here is a possible solution:

var filteredItems =
                string.IsNullOrWhiteSpace(Model)
                    ? ItemList
                    : ItemList.Where(x => x.ModelNumber.ToLowerInvariant().Contains(Model.ToLowerInvariant())).ToList();

Hope this helps, good luck and next time please try to be more specific, or show us more code.

Cheers!

var output=myList.FirstOrDefault(x => X.ModelNumber != null ? x.ModelNumber.ToLower().Contains(Model.ToLower()) : x.ModelNumber))