C＃-使用lambda将表达式参数转换为另一个表达式？

Question

As part of a WPF application I'm building an expression tree and generating a Predicate to use as a filter. The code looks something like this:

public Expression BuildExpression(Expression parameter, string value)
{
    MethodInfo toStringMethod = new Func<Object, string>((a) => a.ToString()).Method;
    Expression lhs = Expression.Call(parameter, toStringMethod );
    ConstantExpression rhs = Expression.Constant(value);
    BinaryExpression result = Expression.Equal(lhs, rhs);
    return result;
}

This is because the parameter is an Expression of unknown type - it might be an int, string, Guid or anything else. The problem is that it's difficult to understand what's going on here without copious comments. I'd really like to use a lambda here:

return parameter => parameter.ToString() == value;

The problem is that this doesn't work as intended - the resulting delegate would call ToString() on the Expression instead of the value of the expression. If it helps, parameter is a MemberExpression.

Answer 1

I was able to find a way to do this with very little extra code. The main inspiration for this solution was Mark Gravell's answer to Combining two expressions (Expression<Func<T, bool>>)

Starting from .net 4.0. There is the ExpressionVistor class which allows you to build expressions that are EF safe.

Expression<TDelegate> is derived from LambdaExpression (if you're interested in learning more about how these actually work I encourage you to check out the "LINQ - Expression Tree Visualizer" sample, currently located at https://code.msdn.microsoft.com/LINQ-Expression-Tree-47608cb5 ). This line of code:

Expression<Predicate<Object>> e1 = a => a.ToString() == "foo";

Creates a LambdaExpression with a single ParameterExpression(Type: Object, ReturnType: Boolean, Body:(LogicalBinaryExpression)). Using a custom ExpressionVisitor we can modify the body of the Lambda to create the same result as my previous code:

public class ReplaceExpressionVisitor
    : ExpressionVisitor
{
    private readonly Expression _oldValue;
    private readonly Expression _newValue;

    public ReplaceExpressionVisitor(Expression oldValue, Expression newValue)
    {
        _oldValue = oldValue;
        _newValue = newValue;
    }

    public override Expression Visit(Expression node)
    {
        if (node == _oldValue)
            return _newValue;
        return base.Visit(node);
    }
}

// Elsewhere...
public Expression BuildExpression(Expression parameter, string value)
{
    Expression<Predicate<Object>> e1 = a => a.ToString() == value;
    return (new ReplaceExpressionVisitor(e1.Parameters[0], parameter)).Visit(e1.Body);
}

Further cleanup is definitely possible (provide a static and/or extension method) but this meets my needs.