I think as you've said that using ExpressionVisitor works out to be a good approach. You don't need to implement all the Visit... methods as they already have a default implementation. From what I understood what you want is to find all property accesses of a certain type inside a lambda function

public class MemberAccessVisitor : ExpressionVisitor

{

    private readonly Type declaringType;

    private IList<string> propertyNames = new List<string>();

    public MemberAccessVisitor(Type declaringType)

    {

        this.declaringType = declaringType;

    }

    public IEnumerable<string> PropertyNames { get { return propertyNames; } }

    public override Expression Visit(Expression expr)

    {

        if (expr != null && expr.NodeType == ExpressionType.MemberAccess)

        {

            var memberExpr = (MemberExpression)expr;

            if (memberExpr.Member.DeclaringType == declaringType)

            {

                propertyNames.Add(memberExpr.Member.Name);

            }

        }

        return base.Visit(expr);

    }

}

This could be further improved to what you want by checking the member is a property and also to get PropertyInfo rather than strings

It could be used as follows:

var visitor = new MemberAccessVisitor(typeof(TSource));

visitor.Visit(memberMap);

var propertyNames = visitor.PropertyNames;