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 problem was two-fold:

• I was misunderstanding how to visit the Lambda expression type. I was still returning a lambda which matched the old delegate instead of returning a new lambda to match the new delegate.

• I needed to hold a reference to the new ParameterExpression instance, which I wasn't doing.

The new code looks like this (notice how the visitor now accepts a reference to a ParameterExpression matching the Entity Framework data object):

class Program
{
    const string conString = @"myDB";

    static void Main(string[] args)
    {
        Expression<Func<string, DateTime, byte, long, bool>> expression = (jobName, ranAt, resultCode, elapsed) => jobName == "Email Notifications" && resultCode == (byte)ResultCode.Failed;
        var criteria = ConvertExpression(expression);

        using (MyDataContext dataContext = new MyDataContext(conString))
        {
            List<svc_JobAudit> jobs = dataContext.svc_JobAudit.Where(criteria).ToList();
        }
    }

    private static Expression<Func<svc_JobAudit, bool>> ConvertExpression(Expression<Func<string, DateTime, byte, long, bool>> expression)
    {
        var jobAuditParameter = Expression.Parameter(typeof(svc_JobAudit), "jobAudit");
        var newExpression = Expression.Lambda<Func<svc_JobAudit, bool>>(
            new ReplaceVisitor()
               .Modify(expression.Body, jobAuditParameter), jobAuditParameter);
        return newExpression;
    }
}

class ReplaceVisitor : ExpressionVisitor
{
    private ParameterExpression parameter;

    public Expression Modify(Expression expression, ParameterExpression parameter)
    {
        this.parameter = parameter;
        return Visit(expression);
    }

    protected override Expression VisitLambda<T>(Expression<T> node)
    {
        return Expression.Lambda<Func<svc_JobAudit, bool>>(Visit(node.Body), Expression.Parameter(typeof(svc_JobAudit)));
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node.Type == typeof(string))
        {
            return Expression.Property(parameter, "JobName");
        }
        else if (node.Type == typeof(DateTime))
        {
            return Expression.Property(parameter, "RanAt");
        }
        else if (node.Type == typeof(byte))
        {
            return Expression.Property(parameter, "Result");
        }
        else if (node.Type == typeof(long))
        {
            return Expression.Property(parameter, "Elapsed");
        }
        throw new InvalidOperationException();
    }
}

Answer 2

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<T>(Expression<T> 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<DateTime,Bool> subst F<Source,DateTime> => F<Source,bool>
        // e.g. F<DateTime,Bool> subst F<Source1,Source2,DateTime> => F<Source1,Source2,bool>

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

        ReadOnlyCollection<ParameterExpression> substitutedParameters
            = new ReadOnlyCollection<ParameterExpression>(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);
    }
}