NJection.LambdaConverter | C # Delegate to LambdaExpression Converter Library | DevOps library

You have to be aware of the difference between an IEnumerable and an IQueryable.

An object that implements IEnumerable represents a sequence: you can get the first element of the sequence, and once you got an element, you can ask for the next element in the sequence.

At its lowest level this is done using GetEnumerator(), and repeatedly MoveNext() / Current. foreach, ToList(), Any(), Count(), etc will deep inside use these methods.

An IQueryable however, does not represent the sequence itself, it represents the potential to create a sequence

To do this, the IQueryable holds an Expression and a Provider. The Expression is a general description of what data must be set. The Provider knows who must provide the data (usually a database management system), and what language is used to communicate with the DBMS (usually SQL).

As long as you concatenate LINQ statements that return IQueryable<...>, you are only changing the Expression. Only if you start enumerating, either by using GetEnumerator / MoveNext, or higher level: foreach, ToList, etc, the Expression is sent to the Provider, who will translate it into SQL and execute the query. The returned data is represented as an enumerable sequence.

The problem is that the Provider does not know your methods GetSourceUOM. GetParentUOM, etc. Therefore it can't translate them into SQL. In fact, there several LINQ methods that also can't be used. See Supported and Unsupported LINQ methods.

What you can do, is convert your extension methods such that it extends IQueryable.