Translating expression tree from a type to another type with complex mappings

Question

inspired by this answer I\'m trying to map a property on a model class to an expression based on the actual entity. These are the two classes involved:

publi         


        

Answer 1

I took the liberty of modifying your code just a hair but this does the trick,

public class Customer
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Id { get; set; }
    public DateTime? BirthDate { get; set; }
    public int CustomerTypeId { get; set; }
}

public class CustomerModel
{
    public string FullName { get; set; }
    public bool HasEvenId { get; set; }
}

sealed class AToBConverter : ExpressionVisitor
{
    private readonly Dictionary _parameters = new Dictionary();
    private readonly Dictionary _mappings;

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node.Type == typeof(TA))
        {
            ParameterExpression parameter;
            if (!this._parameters.TryGetValue(node, out parameter))
            {
                this._parameters.Add(node, parameter = Expression.Parameter(typeof(TB), node.Name));
            }
            return parameter;
        }
        return node;
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Expression == null || node.Expression.Type != typeof(TA))
        {
            return base.VisitMember(node);
        }
        Expression expression = this.Visit(node.Expression);
        if (expression.Type != typeof(TB))
        {
            throw new Exception("Whoops");
        }
        LambdaExpression lambdaExpression;
        if (this._mappings.TryGetValue(node.Member, out lambdaExpression))
        {
            return new SimpleExpressionReplacer(lambdaExpression.Parameters.Single(), expression).Visit(lambdaExpression.Body);
        }
        return Expression.Property(
            expression,
            node.Member.Name
        );
    }

    protected override Expression VisitLambda(Expression node)
    {
        return Expression.Lambda(
            this.Visit(node.Body),
            node.Parameters.Select(this.Visit).Cast()
        );
    }

    public AToBConverter(Dictionary mappings)
    {
        this._mappings = mappings;
    }
}

sealed class SimpleExpressionReplacer : ExpressionVisitor
{
    private readonly Expression _replacement;
    private readonly Expression _toFind;

    public override Expression Visit(Expression node)
    {
        return node == this._toFind ? this._replacement : base.Visit(node);
    }

    public SimpleExpressionReplacer(Expression toFind, Expression replacement)
    {
        this._toFind = toFind;
        this._replacement = replacement;
    }
}

class Program 
{
    private static Dictionary GetMappings()
    {
        var mappings = new Dictionary();
        var mapping = GetMappingFor(model => model.HasEvenId, customer => (customer.Id % 2) == 0);
        mappings.Add(mapping.Item1, mapping.Item2);
        mapping = GetMappingFor(model => model.FullName, customer => customer.FirstName + " " + customer.LastName);
        mappings.Add(mapping.Item1, mapping.Item2);
        return mappings;
    }

    private static Tuple GetMappingFor(Expression> fromExpression, Expression> toExpression)
    {
        return Tuple.Create(((MemberExpression)fromExpression.Body).Member, (LambdaExpression)toExpression);
    }

    static void Main()
    {
        Expression> source = model => model.HasEvenId && model.FullName == "John Smith";
        Expression> desiredResult = model => (model.Id % 2) == 0 && (model.FirstName + " " + model.LastName) == "John Smith";
        Expression output = new AToBConverter(GetMappings()).Visit(source);
        Console.WriteLine("The two expressions do {0}match.", desiredResult.ToString() == output.ToString() ? null : "not ");
        Console.ReadLine();
    }
}