Question

I’m drawing inspiration from this question:

Convert Linq to Sql Expression to Expression Tree

The original poster asked how to convert this to an Expression tree and got a good answer which can be seen in the above link.

List<Region> lst = (from r in dc.Regions
                    where r.RegionID > 2 && r.RegionDescription.Contains("ern")
                    select r).ToList();

How would I got about making a property with a get method that returns a bool that uses the ExpressionTree? I’d like to be able to do something like this (obviously I don’t need the == true):

List<Region> lst = (from r in dc.Regions
                    where (r.AwesomeProperty == true)
                    select r).ToList();

How would I go about defining AwesomeProperty?

Answer 1

You would define AwesomeProperty just like any other property on your LINQ to SQL object. Assuming it is typed as bool (since you compare it to true), you would build the Where query like this:

// Build the parameter to the where clause predicate and access AwesomeProperty

var regionParameter = Expression.Parameter(typeof(Region), "region");

var awesomeProperty = Expression.Property(regionParameter, "AwesomeProperty");

// Build the where clause predicate using the AwesomeProperty access

var predicate = Expression.Lambda<Func<Region, bool>>(awesomeProperty);

// Get the table, which serves as the base query

var table = dc.Regions.AsQueryable();

// Call the Where method using the predicate and the table as the base query

var whereCall = Expression.Call(
    typeof(Queryable),
    "Where",
    new[] { table.ElementType },
    table.Expression,
    predicate);

// Get an IQueryable<Region> which executes the where call on the table

var query = table.Provider.CreateQuery<Region>(whereCall);

var results = query.ToList();