I'm trying to create a generic class to be used to compose queries for Entity Framework (5).
I got it to work, the only problem is that the value is injected as a constant of the query instead of as a parameter. This reduces the possibilities for EF to cache the query and reuse it later on.
This is what I got so far.
public class MinDateFilter<T> : IFilter<T> where T : class
{
private readonly Expression<Func<T, bool>> _predicate;
public MinDateCandidateFilter(Expression<Func<T, DateTime>> propertySelector, DateTime from)
{
from = from.Date.AddDays(-1);
from = new DateTime(from.Year, from.Month, from.Day, 23, 59, 59, 999);
Expression value = Expression.Constant(from, typeof(DateTime));
//ParameterExpression variable = Expression.Variable(typeof(DateTime), "value");
MemberExpression memberExpression = (MemberExpression)propertySelector.Body;
ParameterExpression parameter = Expression.Parameter(typeof(T), "item");
Expression exp = Expression.MakeMemberAccess(parameter, memberExpression.Member);
Expression operation = Expression.GreaterThan(exp, value);
//Expression operation = Expression.GreaterThan(exp, variable);
_predicate = Expression.Lambda<Func<T, bool>>(operation, parameter);
}
public IQueryable<T> Filter(IQueryable<T> items)
{
return items.Where(_predicate);
}
}
this class can be used in two ways:
by sub-classing it:
public class MinCreationDateCandidateFilter : MinDateFilter<Candidate>
{
public MinCreationDateCandidateFilter(DateTime @from) : base(c => c.CreationDate, @from) {}
}
or simply by instantiating it:
var filter = new MinDateFilter<Entities.Transition>(t => t.Date, from.Value);
This is what I managed to achieve so far:
SELECT
[Extent1].[Id] AS [Id]
-- Other fields
FROM [dbo].[Candidates] AS [Extent1]
WHERE [Extent1].[CreationDate] > convert(datetime2, '1982-12-09 23:59:59.9990000', 121)
instead of
SELECT
[Extent1].[Id] AS [Id]
-- Other fields
FROM [dbo].[Candidates] AS [Extent1]
WHERE [Extent1].[CreationDate] > @p__linq__0
If I uncomment the two commented lines and I comment the two above, I get an error saying that the parameter "value" isn't bound.
I hope I gave all the useful details :)
When a parameter is passed as a ConstantExpression
, like this:
Expression.Constant(myString)
... it will produce a fixed, constant symbol on the resulting query:
SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Bar] AS [Bar],
FROM [dbo].[Foo] AS [Extent1]
WHERE [Extent1].[Bar] = "Some text"
If you use some tool to analyze an expression like (f => f.Bar == myString))
like I did with Expression Tree Visualizer, you see that the parameter is actually a MemberExpression
.
So, if you want a parameter on your resulting query, you have to pass something like a property of an object, or the more convenient anonymous type:
Expression.Property(
Expression.Constant(new { Value = myString }),
"Value"
)
This way you're passing a property of the just-created object and your expression tree gets a MemberExpression
, resulting in the following CommandText
:
SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Bar] AS [Bar],
FROM [dbo].[Foo] AS [Extent1]
WHERE [Extent1].[Bar] = @p__linq__0
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