Expression trees - substitute expression

Question

Consider two expression trees:

Expression<Func<float, float, float>> f1 = (x, y) => x + y;
Expression<Func<float, float>> f2 = x => x * x;

I want to substitute the expression f2 as a second parameter of f1 and obtain the following expression:

Expression<Func<float, float, float>> f3 = (x, y) => x + y * y;

The simplest way is to use Expression.Lambda and Expression.Invoke, but the result will look like

(x, y) => f1(x, f2(y))

But this is unacceptable for me due to ORM limitations that cannot handle invoke/lambda properly.

Is it possible to construct the expression without full traversal of expression trees? A working example that fulfill my needs can be found here but I want simpler solution.

Answer 1

You cannot do it without full traversal of both expressions. Fortunately, ExpressionVisitor makes full traversal really easy:

class ReplaceParameter : ExpressionVisitor {
    private readonly Expression replacement;
    private readonly ParameterExpression parameter;
    public ReplaceParameter(
        ParameterExpression parameter
    ,   Expression replacement
    ) {
        this.replacement = replacement;
        this.parameter = parameter;
    }
    protected override Expression VisitParameter(ParameterExpression node) {
        return node == parameter ? replacement : node;
    }
}

Use this visitor twice to complete the replacement:

Expression<Func<float,float,float>> f1 = (x, y) => x + y;
Expression<Func<float,float>> f2 = x => x * x;
var pX = f2.Parameters[0];
var pY = f1.Parameters[1];
var replacerF2 = new ReplaceParameter(pX, pY);
var replacerF1 = new ReplaceParameter(pY, replacerF2.Visit(f2.Body));
var modifiedF1 = Expression.Lambda(
    replacerF1.Visit(f1.Body)
,   f1.Parameters
);
Console.WriteLine(modifiedF1);

The above prints

(x, y) => (x + (y * y))

Demo.