I have built a repository using Lambda expressions to filter my entity collections. As a parameter to the method I'm sending Expression<Func<Case, bool>> exp
. But inside the method I would like to update that same expression with some global filters. I can see that the expression object itself got an Update method, but I can't figure out how it is implemented (and cannot find anything while searching the net).
exp.Update(exp.Body, ???);
Could anyone give an example??
EDIT: Definition of the method: http://msdn.microsoft.com/en-us/library/ee378255.aspx
EDIT2: This is my code (where I try to use .And):
Expression<Func<Case, bool>> newExp = c => c.CaseStatusId != (int)CaseStatus.Finished
var binExp = Expression.And(exp.Body, newExp.Body);
ParameterExpression paramExp = Expression.Parameter(typeof(Expression<Func<Case, bool>>), "c");
return repository.Where(Expression.Lambda<Expression<Func<Case, bool>>>(binExp,
new[] { paramExp }).Compile()).ToArray();
It fails with the following ArgumentException: Lambda type parameter must be derived from System.Delegate
I don't think the Update
method can help you here. It only creates a new lambda, but does not update the original parameters with the new one, you have to do it manually.
I'd recommend having a visitor that replaces the parameter, then you can And
the expressions together.
In total you would get something like:
private Case[] getItems(Expression<Func<Case, bool>> exp)
{
return repository.Where(AddGlobalFilters(exp).Compile()).ToArray();
}
private Expression<Func<Case, bool>> AddGlobalFilters(Expression<Func<Case, bool>> exp)
{
// get the global filter
Expression<Func<Case, bool>> newExp = c => c.CaseStatusId != (int)CaseStatus.Finished;
// get the visitor
var visitor = new ParameterUpdateVisitor(newExp.Parameters.First(), exp.Parameters.First());
// replace the parameter in the expression just created
newExp = visitor.Visit(newExp) as Expression<Func<Case, bool>>;
// now you can and together the two expressions
var binExp = Expression.And(exp.Body, newExp.Body);
// and return a new lambda, that will do what you want. NOTE that the binExp has reference only to te newExp.Parameters[0] (there is only 1) parameter, and no other
return Expression.Lambda<Func<Case, bool>>(binExp, newExp.Parameters);
}
/// <summary>
/// updates the parameter in the expression
/// </summary>
class ParameterUpdateVisitor : ExpressionVisitor
{
private ParameterExpression _oldParameter;
private ParameterExpression _newParameter;
public ParameterUpdateVisitor(ParameterExpression oldParameter, ParameterExpression newParameter)
{
_oldParameter = oldParameter;
_newParameter = newParameter;
}
protected override Expression VisitParameter(ParameterExpression node)
{
if (object.ReferenceEquals(node, _oldParameter))
return _newParameter;
return base.VisitParameter(node);
}
}
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