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POHow to compile an Expression down to the actual result?
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copied!<p>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.</p> <p>For example, I can do something like this which will retrieve all checking accounts with a bank name of Bank 1 or Bank 2: </p> <p>"bankname = 'Bank 1' or bankname = 'Bank 2'"</p> <p>The web service can handle significantly more complex queries but I'll just stick with this for now.</p> <p>So I have a class CheckingAccount:</p> <pre><code> [IntacctObject("checkingaccount")] public class CheckingAccount : Entity { [IntacctElement("bankaccountid")] public string Id { get; set; } [IntacctElement("bankname")] public string BankName { get; set; } } </code></pre> <p>And an ExpressionVisitor whose primary function is to convert an expression like this:</p> <pre> Expression> expression = ca => ca.BankName == "Bank 1" || ca.BankName == "Bank 2" </pre> <p>into the query: "bankname = 'Bank 1' or bankname = 'Bank 2'"</p> <p>This isn't so tough. Where things start to break down are when I introduce local variables:</p> <pre><code> var account = new CheckingAccount { BankName = "Bank 1" }; string bankName = "Bank 2"; Expression> expression = ca => ca.BankName == account.BankName || ca.BankName == bankName; </code></pre> <p>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.</p> <p>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.</p> <p>Here is my expression visitor (please note the generic constraint on method CreateFilter):</p> <pre><code> 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; } } </code></pre> <p>Please let me know if you'd like any more detail....</p> <p>Thanks</p>

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