Allow ExpressionType.Convert MemberExpressions

[mieliespoor]

I am currently using the initial SQLite-net source, but I just checked the source on this project and see that it is the same.

On the TableQuery CompileExpr method, MemberExpressions only check for ExpressionType.Parameter. If the ExpressionType is Convert, an exception is thrown. Is there a reason for this since if the check is changed to the following, it still works in a scenario that I need it for - if (mem.Expression!=null && (mem.Expression.NodeType == ExpressionType.Parameter || mem.Expression.NodeType == ExpressionType.Convert)) {

This is also the case in the AddOrderBy method. I assume in other places also, but for now it is the only places that I picked it up according to my code.

A sample method that I use to replicate my issue:

public async Task<DateTime> GetLatestModifiedDateAsync<T>() where T : IEntity, new()
{
    var result = await db.Table<T>()
                         .OrderByDescending(e => e.ModifiedAt)
                         .FirstOrDefaultAsync();

    return result != null ? result.ModifiedAt : new DateTime(1900, 1, 1);
}

[mieliespoor]

I have played around with this a bit more since I saw that a specific query of mine does not return the correct result the query. This query would always return 1 if records exist in the table:

var doesexist = await db.Table<T>()
                            .Where(x => x.ObjectId == entity.ObjectId)
                            .CountAsync();

The problem comes in with the CompileExpr method that does not properly compile the Expression into a query.
I have subsequently made the following change which results in the Expression being compiled correctly:

Change this section:
else if (expr.NodeType == ExpressionType.MemberAccess) {
and added:

if (mem.Expression is UnaryExpression && ((mem.Expression as UnaryExpression).Operand.NodeType != ExpressionType.Parameter))
            {
                return CompileExpr(mem.Expression, queryArgs);
            }

This all works except that where the UnaryExpression is being compiled, the CompileResult fails because of the ConvertTo method.

The ConvertTo fails because the value of valr.Value is the entity and the value of ty is IEntity. ConvertTo then calls Convert.ChangeType(obj, t) which fails because entity does not implement IConvertible. I won't be able to implement IConvertible, because it's not available in WinRT.

I am thus stuck. What is the best way to go about to get this working?