常规 LINQ 方法链接中间的表达式树； 动态和可空类型？

Question

I'm trying to write a dynamic query but having trouble when dealing with dynamic types in the OrderBy section of my query. I've seen some solutions using method call expressions, which may work for me, but I don't know how to fit that into my existing code where I am using method chaining on either side. It's possible I can just use dynamic as my type but I don't think that will work with Nullable types.

public async Task<IEnumerable<MyEntityModel>> GetEntities(QueryEntityResource request)
{
    IQueryable<MyEntityModel> queryableData = ...;
    Expression whereLambdaExp = ...;

    queryableData = queryableData.Where(whereLambdaExp);

    ParameterExpression param = Expression.Parameter(typeof(MyEntityModel));
    PropertyInfo property = typeof(MyEntityModel).GetProperty(request.sortModel.ColId);
    Expression propExpr = Expression.Property(param, property);
    var lambdaExpression = Expression.Lambda<Func<MyEntityModel, dynamic>>(propExpr , new ParameterExpression[] { param }); // can't use dynamic

    // Errors here; can't use dynamic type in lambda expression -- Need to specify types
    if (request.sortModel.Sort == "asc")
    {
        queryableData = queryableData.OrderBy(lambdaExpression);
    }
    if (request.sortModel.Sort == "desc")
    {
        queryableData = queryableData.OrderByDescending(lambdaExpression);
    }

    queryableData = queryableData.Skip(request.StartRow).Take(request.EndRow - request.StartRow);

    return queryableData.ToListAsync();
}

If I use dynamic it won't work with my nullable types:

System.ArgumentException: Expression of type 'System.Nullable`1[System.Int32]' cannot be used for return type 'System.Object'

Answer 1

This is untested; but I think it is close to what you need (I feel like I've written this code before... also, see this answer ):

You can build calls to OrderBy instantiated to the types you need and then build a new IQueryable . My method removes the async as I couldn't build a quick test for that in LINQPad.

public IEnumerable<MyEntityModel> GetEntities(QueryEntityResource request) {
    IQueryable<MyEntityModel> queryableData = default;
    Expression<Func<MyEntityModel, bool>> whereLambdaExp = default;

    queryableData = queryableData.Where(whereLambdaExp);

    var param = Expression.Parameter(typeof(MyEntityModel));
    var propExpr = Expression.Property(param, request.sortModel.ColId);
    var lambdaExpression = Expression.Lambda(propExpr, param);

    if (request.sortModel.Sort == "asc" || request.sortModel.Sort == "desc")
        queryableData = queryableData.Provider.CreateQuery<MyEntityModel>(Expression.Call(typeof(Queryable), request.sortModel.Sort == "asc" ? "OrderBy" : "OrderByDescending",
                        new[] { typeof(MyEntityModel), propExpr.Type }, queryableData.Expression, Expression.Quote(lambdaExpression)));

    queryableData = queryableData.Skip(request.StartRow).Take(request.EndRow - request.StartRow);

    return queryableData.ToList();
}