How to create dynamic condition using Expression Tree
c# c#-4.0 entity-framework expression-trees linq
Question
Please consider this code:
System.Linq.Expressions.Expression<Func<tbl, bool>> exp_details = r => r.ID_Master == Id &&
r.Year == Year &&
r.Month == Month ;
I want to write a function that expect some argument, and then retrieve some data from my database. The problem is I want to create dynamic condition.dor example if I pass IsDeleted argument with true value I want to add r.IsDeleted == true; to exp_details. How can I do this?
Accepted Answer
The key to doing this is to use an ExpressionVisitor which walks the given expression and (optionally, in a subclass) allows for replacing recognized elements with others you specify. In my case this was done for ORM (NHibernate). This is what I use: (I'll add references to my answer later)
public class ParameterAssignAndReplacer : ExpressionVisitor
{
private readonly ParameterExpression _source;
private readonly ConstantExpression _target;
internal ParameterAssignAndReplacer(ParameterExpression source, ConstantExpression target)
{
_source = source;
_target = target;
}
protected override Expression VisitParameter(ParameterExpression node)
{
return node.Name == _source.Name ?
base.VisitConstant(_target) :
base.VisitParameter(node);
}
}
And..
public static class ExpressionExtensions
{
public static Expression<Func<TArg2, TResult>> AssignAndReduce<TArg1, TArg2, TResult>(
this Expression<Func<TArg1, TArg2, TResult>> func,
TArg1 parameter)
{
var exprBody = func.Body;
var assignedParameter = Expression.Constant(parameter, typeof(TArg1));
exprBody = new ParameterAssignAndReplacer(func.Parameters[0], assignedParameter).Visit(exprBody);
return Expression.Lambda<Func<TArg2, TResult>>(exprBody, func.Parameters[1]);
}
}
Both classes can be extended to your specific scenario. To make this relevant to the code you posted (I'm using only IsDeleted as the parameter for simplicity):
public class SomeClass
{
Expression<Func<tbl, bool, bool>> _templateExpression =
(tbl r, bool isDeleted) => r.ID_Master == 5 && r.Year == 2008 && r.Month == 12 && r.IsDeleted == isDeleted;
public Expression<Func<tbl, bool>> Foo(bool IsDeleted)
{
return _templateExpression.AssignAndReduce(IsDeleted);
}
}
Regarding references, most of what I learned on this topic is from Marc Gravell's "Expressions" answers [ although many other users have helped me get a grip on this :-) ]
Popular Answer
you can't use statement body to Linq Expression consider using Predicate instead
var exp_details = new Predicate<tbl>(r =>
{
bool result == Id && r.Year == Year && r.Month == Month;
if(IsDeleted != null)
{
result &= r.IsDeleted == IsDeleted;
}
return result;
});
the most Linq Func<T, bool> expression can be replaced with Predicate<T>.
To generate dynamic WHERE cause using "simple" linq expression.
IQueryable<tbl> query = ent.tbl.Where(r => r.ID_Master == Id && r.Year == Year);
//customize query
if(IsDeleted != null){
query = query.Where(r => r.IsDeleted == IsDeleted);
}
//execute the final generated query
var result = query.FirstOrDefault();
this will create where cause from IQueryable<T>. Linq is smart enough for complex query.