Question

How would I go about joining two lambda expressions like theese:

Expression<Func<string, bool>> expr1 = a => a.Length > 100;
Expression<Func<string, bool>> expr2 = b => b.Length < 200;

… into an expression like this:

Expression<Func<string, bool>> expr3 = s => s.Length < 100 && s.Length < 200;

That is, joining them with an AndAlso operator. (Or any other operator for that matter…)

I actually succeeded with some nasty recursive replacement of lambda parameters and then joining with the Expression.AndAlso method. But I’m looking for something more simple.

For example something like: (Which obviously doesn’t work.)

Expression<Func<string, bool>> expr3 = c => expr1(a) && expr2(b);

Answer 1

Your “something like” would work if you were dealing with normal delegates.
But if you have to use expression trees, I don’t see any other solution than recursive replacement.

In .NET 4, you can use the System.Linq.Expressions.ExpressionVisitor to make this kind of recursive replacement much easier. For .NET 3.5, take a look at this sample: http://msdn.microsoft.com/en-us/library/bb882521.aspx

Using the ExpressionVisitor, you only have to override methods for the node types you want to replace and the surrounding tree will be automatically reconstructed.

If you are dealing with conditions for use with LINQ, a much easier solution to dynamically combine conditions is to just call Where() multiple times.