How to compile an Expression down to the actual re

问题:

I am building an API around a web service call using Expressions to allow a developer to specify a query and have an ExpressionVisitor convert the Expression into the query string. The request is XML with a particular element containing a query string.

For example, I can do something like this which will retrieve all checking accounts with a bank name of Bank 1 or Bank 2:

"bankname = 'Bank 1' or bankname = 'Bank 2'"

The web service can handle significantly more complex queries but I'll just stick with this for now.

So I have a class CheckingAccount:

[IntacctObject("checkingaccount")]
public class CheckingAccount : Entity
{
    [IntacctElement("bankaccountid")]
    public string Id { get; set; }

    [IntacctElement("bankname")]
    public string BankName { get; set; }
}

And an ExpressionVisitor whose primary function is to convert an expression like this:

Expression> expression = ca => ca.BankName == "Bank 1" || ca.BankName == "Bank 2"

into the query: "bankname = 'Bank 1' or bankname = 'Bank 2'"

This isn't so tough. Where things start to break down are when I introduce local variables:

var account = new CheckingAccount { BankName = "Bank 1" };
string bankName = "Bank 2";

Expression> expression = ca => ca.BankName == account.BankName || ca.BankName == bankName;

I know how to deal with a simple local variable (ie. string bankName = "Bank 2") but dealing with a the other type (var account = new CheckingAccount { BankName = "Bank 1" }) is much more complex.

At the end of the day these are the big issues that I need to figure out how to deal with right now. I know there are much more complex scenarios but I'm not so concerned with those at the moment.

Here is my expression visitor (please note the generic constraint on method CreateFilter):

internal class IntacctWebService30ExpressionVisitor : ExpressionVisitor
{
    private readonly List _Filters = new List();
    private IntacctWebServicev30SimpleQueryFilter _CurrentSimpleFilter;
    private IntacctWebServicev30ComplexQueryFilter _CurrentComplexFilter;
    private MemberExpression _CurrentMemberExpression;

    public string CreateFilter(Expression> expression) where TEntity : Entity
    {

        Visit(expression);

        string filter = string.Join(string.Empty, _Filters.Select(f => f.ToString()).ToArray());
        return filter;
    }

    protected override Expression VisitBinary(BinaryExpression node)
    {
        switch (node.NodeType)
        {
            case ExpressionType.AndAlso:
            case ExpressionType.OrElse:
                _CurrentComplexFilter = new IntacctWebServicev30ComplexQueryFilter { ExpressionType = node.NodeType };
                break;
            case ExpressionType.Equal:
            case ExpressionType.GreaterThan:
            case ExpressionType.GreaterThanOrEqual:
            case ExpressionType.LessThan:
            case ExpressionType.LessThanOrEqual:
            case ExpressionType.NotEqual:
                _CurrentSimpleFilter = new IntacctWebServicev30SimpleQueryFilter { ExpressionType = node.NodeType };
                break;
        }

        return base.VisitBinary(node);
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        var attr = node.Member.GetAttribute();
        if (attr != null)
            _CurrentSimpleFilter.FieldName = attr.FieldName;
        else
            _CurrentMemberExpression = node;

        return base.VisitMember(node);
    }

    protected override Expression VisitConstant(ConstantExpression node)
    {
        object value = Expression.Lambda>(node).Compile().Invoke();

        string fieldValue = extraxtFieldValue(value, node);

        _CurrentSimpleFilter.FieldValue = fieldValue;

        if (_CurrentComplexFilter != null)
        {
            if (_CurrentComplexFilter.Left == null)
            {
                _CurrentComplexFilter.Left = _CurrentSimpleFilter;
            }
            else if (_CurrentComplexFilter.Right == null)
            {
                _CurrentComplexFilter.Right = _CurrentSimpleFilter;
                _Filters.Add(_CurrentComplexFilter);
            }
            else
                throw new InvalidOperationException();
        }
        else
        {
            _Filters.Add(_CurrentSimpleFilter);
        }

        return base.VisitConstant(node);
    }

    private string extraxtFieldValue(object value)
    {
        string fieldValue;
        if (value is DateTime)
            fieldValue = ((DateTime)value).ToString("yyyy-MM-dd");
        else if (value is string)
            fieldValue = value.ToString();
        else if (value.GetType().IsEnum)
        {
            throw new NotImplementedException();
        }
        else
        {
            // Not sure if this is the best way to do this or not but can't figure out how
            // else to get a variable value.

            // If we are here then we are dealing with a property, field, or variable.
            // This means we must extract the value from the object.
            // In order to do this we will rely on _CurrentMemberExpression
            if (_CurrentMemberExpression.Member.MemberType == MemberTypes.Property)
            {
                fieldValue = value.GetType().GetProperty(_CurrentMemberExpression.Member.Name).GetValue(value, null).ToString();
            }
            else if (_CurrentMemberExpression.Member.MemberType == MemberTypes.Field)
            {
                fieldValue = value.GetType().GetFields().First().GetValue(value).ToString();
            }
            else
            {
                throw new InvalidOperationException();
            }
        }

        return fieldValue;
    }
}

Please let me know if you'd like any more detail....

Thanks

回答1:

Have a look at an article about this very issue from Matt Warren. He provides source for a class that does exactly this, with an explanation how does it do it. It's also included in his IQToolkit.

回答2:

If you are interested in using a open source third party library to do this for you, you can check out Expression Tree Serialization. I believe it does what you are looking for.