如何在二进制表达式中将参数表达式更改为常数表达式

Question

I have my custom Visitor which looks to right and left and changes parameters to constant.
I know that just change node is not possible.
I should return new lambda expression which contains constants instead parameters. But I can not create an expression myself :(
I have this code:

    public class ParametersTransformToConstantVisitor : ExpressionVisitor
    {

    private Dictionary<string, ConstantExpression> parameters = new Dictionary<string, ConstantExpression>();

    public ParametersTransformToConstantVisitor(Dictionary<string, ConstantExpression> parameters)
    {
        this.parameters = parameters;
    }

    protected override Expression VisitBinary(BinaryExpression node)
    {
        var constExprLeftName = new Lazy<string>(() => ((ParameterExpression) node.Left)?.Name);
        var constExprRightName = new Lazy<string>(() => ((ParameterExpression) node.Right)?.Name);
        var constExprName = new Lazy<string>(() => ((ParameterExpression) node.Reduce())?.Name);

        ParameterExpression leftParam = null;
        ParameterExpression rightParam = null;

        if (node.NodeType == ExpressionType.Parameter && parameters.ContainsKey(constExprName.Value))
        {
            return parameters[constExprName.Value];
        }

        if (node.Left.NodeType == ExpressionType.Parameter && parameters.ContainsKey(constExprLeftName.Value))
        {
            leftParam = (ParameterExpression) node.Left;
        }       

        if (node.Right.NodeType == ExpressionType.Parameter && parameters.ContainsKey(constExprLeftName.Value))
        {
            rightParam = (ParameterExpression) node.Right;
        }

        if (leftParam != null || rightParam != null)
        {
            //return Expression.Lambda();
        }       

        return base.VisitBinary(node);
    }
}

Help me to build lambda expression, please

Answer 1

It feels like all you actually need here is:

protected override Expression VisitParameter(ParameterExpression node)
    => parameters.TryGetValue(node.Name, out var ce) ? (Expression)ce : node;

protected override Expression VisitLambda<T>(Expression<T> node)
    => Expression.Lambda(Visit(node.Body), node.Parameters); // don't visit the parameters

ie whenever the visitor sees a ParameterExpression , if there is a corresponding item in the parameters map, use that value.

The override on VisitLambda is because VisitLambda still needs to return a lambda of the same shape , and the default implementation would also visit (and thus swap) out the parameters from the declaration .

It is the visitor's job to worry about reassembling the tree around your changes.

Note, however, that if you are trying to create a parameterless lambda , you might also need to rewrite the root. Or you could just use the .Body and forget about the parameters.

Example:

Expression<Func<int, int, string>> add = (x, y) => ((2 * x) + y).ToString();
Console.WriteLine(add);

var args = new Dictionary<string, ConstantExpression>
{
    ["x"] = Expression.Constant(4),
    ["y"] = Expression.Constant(1),
};

var visitor = new ParametersTransformToConstantVisitor(args);
var result = (LambdaExpression)visitor.Visit(add);
Console.WriteLine(result);

which gives:

(x, y) => ((2 * x) + y).ToString()
(x, y) => ((2 * 4) + 1).ToString()

You can make this into a parameterless lambda via:

var withoutArgs = Expression.Lambda<Func<string>>(result.Body);
Console.WriteLine(withoutArgs);

which gives:

() => ((2 * 4) + 1).ToString()

---

minor addition: you might also want to simplify in the visitor:

protected override Expression VisitBinary(BinaryExpression node)
{
    var visited = base.VisitBinary(node);

    if(visited is BinaryExpression be
        && be.Method == null && be.Conversion == null
        && !be.IsLifted
        && be.Left is ConstantExpression left
        && be.Right is ConstantExpression right)
    {

        object val;
        switch(be.NodeType)
        {
            case ExpressionType.Add:
                val = (dynamic)left.Value + (dynamic)right.Value;
                break;
            case ExpressionType.Multiply:
                val = (dynamic)left.Value * (dynamic)right.Value;
                break;
            case ExpressionType.Subtract:
                val = (dynamic)left.Value - (dynamic)right.Value;
                break;
            case ExpressionType.Divide:
                val = (dynamic)left.Value / (dynamic)right.Value;
                break;
            default:
                return visited; // unknown
        }

        return Expression.Constant(
            Convert.ChangeType(val, visited.Type), visited.Type);
    }
    return visited;
}

This changes the outputs to:

(x, y) => ((2 * x) + y).ToString()
(x, y) => 9.ToString()
() => 9.ToString()

and we could possibly also even hoist the ToString() !

protected override Expression VisitMethodCall(MethodCallExpression node)
{
    var visited = base.VisitMethodCall(node);
    if (visited is MethodCallExpression mce)
    {
        if ((mce.Object == null || mce.Object is ConstantExpression)
            && mce.Arguments.All(x => x is ConstantExpression))
        {
            var obj = (mce.Object as ConstantExpression)?.Value;
            var args = mce.Arguments.Select(x => ((ConstantExpression)x).Value).ToArray();
            var result = mce.Method.Invoke(obj, args);
            return Expression.Constant(result, mce.Type);
        }
    }
    return visited;
}

which now gives us:

(x, y) => ((2 * x) + y).ToString()
(x, y) => "9"
() => "9"