Question

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

Answer 1

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);
        }
    }