Force VB.NET to generate the same string comparison expression as C#?

问题

Somewhat similar question here:

Difference between C# and VB.Net string comparison

...but not the same as the one I am asking now.

I am creating a simple expression walker that will convert a lambda into an SQL WHERE clause. I call it like this:

GetEntities<MyEntity>(e => e.MyProperty == MyValue)

C# creates the expression as I would expect which is a BinaryExpression consisting of a MemberExpression on the left and a ConstantExpression on the right which looks like this:

$e.MyProperty == MyValue

VB.NET, however, calls CompareString, to which it passes MyProperty and MyValue as parameters and then checks the return result for 0. When called like this:

GetEntities(Of MyEntity)(Function(e) e.MyProperty = MyValue)

...it generates an expression like this:

.Call Microsoft.VisualBasic.CompilerServices.Operators.CompareString(
    $e.MyProperty, MyValue, False) == 0

This obviously doesn't play too well with my expression walker, so I will have to now walk the method expression to get the value passed into it and blah blah blah.

Is there a way to force VB.NET to generate the same expression tree as C# in all circumstances? I'd hate to have to write a ton of code to account for these significant differences.

回答1:

As I noted in a comment there is a reason for the difference. The Vb.Net operators does not work in the same way as in C#.

So the answer to your questions is no, you can't change Vb.Net's operators to work like C#.

What you can do is to create a transform from Vb expression tree to C# expression tree:

    internal sealed class VbComparisonTransform : ExpressionVisitor
    {
        protected override Expression VisitBinary(BinaryExpression node) {
            if (node == null)
                throw new ArgumentNullException("node");

            if (node.Left.NodeType != ExpressionType.Call)
                return base.VisitBinary(node);

            var callNode = node.Left as MethodCallExpression;
            if (callNode.Method.DeclaringType.FullName != "Microsoft.VisualBasic.CompilerServices.Operators")
                return base.VisitBinary(node);
            if (callNode.Method.Name != "CompareString")
                return base.VisitBinary(node);

            switch (node.NodeType) {
                case ExpressionType.LessThan:
                    return Expression.LessThan(callNode.Arguments[0], callNode.Arguments[1]);
                case ExpressionType.LessThanOrEqual:
                    return Expression.LessThanOrEqual(callNode.Arguments[0], callNode.Arguments[1]);
                case ExpressionType.Equal:
                    return Expression.Equal(callNode.Arguments[0], callNode.Arguments[1]);
                case ExpressionType.NotEqual:
                    return Expression.NotEqual(callNode.Arguments[0], callNode.Arguments[1]);
                case ExpressionType.GreaterThanOrEqual:
                    return Expression.GreaterThanOrEqual(callNode.Arguments[0], callNode.Arguments[1]);
                case ExpressionType.GreaterThan:
                    return Expression.GreaterThan(callNode.Arguments[0], callNode.Arguments[1]);
                default:
                    string throwmessage = string.Format(CultureInfo.InvariantCulture, "VB.Net compare expression of type {0} not supported", node.NodeType);
                    throw new NotSupportedException(throwmessage);
            }
        }
    }

and then use it like this:

public Expression ToCSharpComparisons(Expression expression) {
    if (expression == null)
        throw new ArgumentNullException("expression");

    return new VbComparisonTransform().Visit(expression);
}

来源：https://stackoverflow.com/questions/35015198/force-vb-net-to-generate-the-same-string-comparison-expression-as-c

标签

vb.net

expression-trees

equality