How to build a LambdaExpression from an existing L

问题:

I want to combine two LambdaExpressions without compiling them.

This is what it looks like if I do compile them:

    public Expression<Func<TContainer,bool>> CreatePredicate<TContainer,TMember>(
        Expression<Func<TContainer,TMember>> getMemberExpression, 
        Expression<Func<TMember,bool>> memberPredicateExpression)
    {
        return x => memberPredicateExpression.Compile()(getMemberExpression.Compile()(x));
    }

That's obviously not the fastest way to get the target expression from the provided arguments. Also, it makes it incompatible with query providers like LINQ to SQL that do not support C# method calls.

From what I've read it seems like the best approach is to build an ExpressionVisitor class. However, this seems like it could be a pretty common task. Does anyone know of an existing open source code base that provides this kind of functionality? If not, what is the best way to approach the ExpressionVisitor to make it as generic as possible?

回答1:

I don't know if it's the best way, but you could do something like that:

public Expression<Func<TContainer,bool>> CreatePredicate<TContainer,TMember>(
    Expression<Func<TContainer,TMember>> getMemberExpression, 
    Expression<Func<TMember,bool>> memberPredicateExpression)
{
    ParameterExpression x = Expression.Parameter(typeof(TContainer), "x");
    return Expression.Lambda<Func<TContainer, bool>>(
        Expression.Invoke(
            memberPredicateExpression,
            Expression.Invoke(
                getMemberExpression,
                x)),
        x);
}

Usage:

var expr = CreatePredicate(
    (Foo f) => f.Bar,
    bar => bar % 2 == 0);

Result:

x => Invoke(bar => ((bar % 2) == 0), Invoke(f => f.Bar, x))

I guess it would be better to get something like x => x.Bar % 2 == 0, but it would probably be significantly harder...

---

EDIT: actually it wasn't so hard with an expression visitor:

public Expression<Func<TContainer,bool>> CreatePredicate<TContainer,TMember>(
    Expression<Func<TContainer,TMember>> getMemberExpression, 
    Expression<Func<TMember,bool>> memberPredicateExpression)
{
    return CombineExpressionVisitor.Combine(
        getMemberExpression,
        memberPredicateExpression);
}

class CombineExpressionVisitor : ExpressionVisitor
{
    private readonly ParameterExpression _parameterToReplace;
    private readonly Expression _replacementExpression;
    private CombineExpressionVisitor(ParameterExpression parameterToReplace, Expression replacementExpression)
    {
        _parameterToReplace = parameterToReplace;
        _replacementExpression = replacementExpression;
    }

    public static Expression<Func<TSource, TResult>> Combine<TSource, TMember, TResult>(
        Expression<Func<TSource, TMember>> memberSelector,
        Expression<Func<TMember, TResult>> resultSelector)
    {
         var visitor = new CombineExpressionVisitor(
            resultSelector.Parameters[0],
            memberSelector.Body);
        return Expression.Lambda<Func<TSource, TResult>>(
            visitor.Visit(resultSelector.Body),
            memberSelector.Parameters);
    }

    protected override Expression VisitParameter(ParameterExpression parameter)
    {
        if (parameter == _parameterToReplace)
            return _replacementExpression;
        return base.VisitParameter(parameter);
    }
}

It gives the following expression:

f => ((f.Bar % 2) == 0)