Using a LINQ ExpressionVisitor to replace primitive parameters with property references in a lambda expression

Question

I\'m in the process of writing a data layer for a part of our system which logs information about automated jobs that run every day - name of the job, how long it ran, what

Answer 1

The accepted answer is 'hardcoded' to some specific types. Here's a more general expression rewriter than can substitute a parameter for any other expression (lambda, constant, ...). In the case of a lambda expression the expression's signature needs to change to incorporate the parameters needed by the substituted value.

public class ExpressionParameterSubstitute : System.Linq.Expressions.ExpressionVisitor
{
    private readonly ParameterExpression from;
    private readonly Expression to;
    public ExpressionParameterSubstitute(ParameterExpression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }

    protected override Expression VisitLambda(Expression node)
    {
        if (node.Parameters.All(p => p != this.from))
            return node;

        // We need to replace the `from` parameter, but in its place we need the `to` parameter(s)
        // e.g. F subst F => F
        // e.g. F subst F => F

        var toLambda = to as LambdaExpression;
        var substituteParameters = toLambda?.Parameters ?? Enumerable.Empty();

        ReadOnlyCollection substitutedParameters
            = new ReadOnlyCollection(node.Parameters
                .SelectMany(p => p == this.from ? substituteParameters : Enumerable.Repeat(p, 1) )
                .ToList());

        var updatedBody = this.Visit(node.Body);        // which will convert parameters to 'to'
        return Expression.Lambda(updatedBody, substitutedParameters);
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        var toLambda = to as LambdaExpression;
        if (node == from) return toLambda?.Body ?? to;
        return base.VisitParameter(node);
    }
}