p => p.Price < 1000 && p.Name.StartsWith("a-string") && !p.OutOfStock

下图标记了它的不同部分：

public class FilterDescriptor
{public FilterDescriptor(){CompositionOperator = FilterOperator.And;}private FilterOperator _compositionOperator;public FilterOperator CompositionOperator{get => _compositionOperator;set{if (value != FilterOperator.And && value != FilterOperator.Or)throw new ArgumentOutOfRangeException();_compositionOperator = value;}}public string FieldName { get; set; }public object Value { get; set; }public FilterOperator Operator { get; set; }     // For demo purposespublic override string ToString(){return$"{CompositionOperator} {FieldName ?? "FieldName"} {Operator} {Value ?? "Value"}";}
}

FilterOperator类的属性类型是一个枚举。此属性指定过滤器的运算符。

public enum FilterOperator
{NOT_SET,// LogicalAnd,Or,Not,// ComparisonEqual,NotEqual,LessThan,LessThanOrEqual,GreaterThan,GreaterThanOrEqual,// StringStartsWith,Contains,EndsWith,NotStartsWith,NotContains,NotEndsWith
}

表达式节点不会直接转换为FilterDescriptor对象。取而代之的是，每个访问表达式节点的重写方法，都创建一个名为token的对象并将其添加到私有列表中。该列表中的令牌是根据逆波兰式（RPN）排列的。什么是令牌？令牌封装了构建FilterDescriptor所需的节点数据。令牌由继承自抽象Token类的类定义。

public abstract class Token {}public class BinaryOperatorToken : Token
{public FilterOperator Operator { get; set; }public BinaryOperatorToken(FilterOperator op){Operator = op;}public override string ToString(){return "Binary operator token:\t" + Operator.ToString();}
}public class ConstantToken : Token
{public object Value { get; set; }public ConstantToken(object value){Value = value;}public override string ToString(){return "Constant token:\t\t" + Value.ToString();}
}public class MemberToken : Token
{public Type Type { get; set; }public string MemberName { get; set; }public MemberToken(string memberName, Type type){MemberName = memberName;Type = type;}public override string ToString(){return "Member token:\t\t" + MemberName;}
}public class MethodCallToken : Token
{public string MethodName { get; set; }public MethodCallToken(string methodName){MethodName = methodName;}public override string ToString(){return "Method call token:\t" + MethodName;}
}public class ParameterToken : Token
{public string ParameterName { get; set; }public Type Type { get; set; }public ParameterToken(string name, Type type){ParameterName = name;Type = type;}public override string ToString(){return "Parameter token:\t\t" + ParameterName;}
}public class UnaryOperatorToken : Token
{public FilterOperator Operator { get; set; }public UnaryOperatorToken(FilterOperator op){Operator = op;}public override string ToString(){return "Unary operator token:\t\t" + Operator.ToString();}
}

遍历表达式的所有节点并创建它们的等效标记后，FilterDescriptor就可以创建。这将通过调用Build名为的方法来完成。

public IEnumerable<FilterDescriptor> Build()
{var filters = new Stack<FilterDescriptor>();for (var i = 0; i < _tokens.Count; i++){var token = _tokens[i];switch (token){case ParameterToken p:var f = getFilter();f.FieldName = p.ParameterName;filters.Push(f);break;case BinaryOperatorToken b:var f1 = getFilter();switch (b.Operator){case FilterOperator.And:case FilterOperator.Or:var ff = filters.Pop();ff.CompositionOperator = b.Operator;filters.Push(ff);break;case FilterOperator.Equal:case FilterOperator.NotEqual:case FilterOperator.LessThan:case FilterOperator.LessThanOrEqual:case FilterOperator.GreaterThan:case FilterOperator.GreaterThanOrEqual:f1.Operator = b.Operator;filters.Push(f1);break;}break;case ConstantToken c:var f2 = getFilter();f2.Value = c.Value;filters.Push(f2);break;case MemberToken m:var f3 = getFilter();f3.FieldName = m.MemberName;filters.Push(f3);break;case UnaryOperatorToken u:var f4 = getFilter();f4.Operator = u.Operator;f4.Value = true;filters.Push(f4);break;case MethodCallToken mc:var f5 = getFilter();f5.Operator = _methodCallMap[mc.MethodName];filters.Push(f5);break;}}var output = new Stack<FilterDescriptor>();while (filters.Any()){output.Push(filters.Pop());}return output;FilterDescriptor getFilter(){if (filters.Any()){var f = filters.First();var incomplete = f.Operator == default ||f.CompositionOperator == default ||f.FieldName == default ||f.Value == default;if (incomplete)return filters.Pop();return new FilterDescriptor();}return new FilterDescriptor();}
}

当Build方法返回时，所有描述符都准备好转换为所需的任何形式。

private class BooleanVisitor : ExpressionVisitor
{protected override Expression VisitMember(MemberExpression node){if (node.Type == typeof(bool)){return Expression.MakeBinary(ExpressionType.Equal, node, Expression.Constant(true));}return base.VisitMember(node);}
}

此重写的方法向其添加布尔成员访问表达式的两个缺失部分，并返回修改后的副本。第二遍需要在之后执行。这是在FilterBuilder的构造函数中完成的。

// ctor of the FilterBuilderpublic FilterBuilder(Expression expression)
{var fixer = new BooleanVisitor();var fixedExpression = fixer.Visit(expression);base.Visit(fixedExpression);
}

修改否定比较运算符

protected override Expression VisitMember(MemberExpression node)
{if (node.Type == typeof(DateTime)){if (node.Expression == null) // Simple MemberAccess like DateTime.Now{var lambda = Expression.Lambda<Func<DateTime>>(node);var dateTime = lambda.Compile()();base.Visit(Expression.Constant(dateTime.Ticks));return node;}else{switch (node.Expression.NodeType){case ExpressionType.New:var lambda = Expression.Lambda<Func<DateTime>>(node.Expression);var dateTime = lambda.Compile()();base.Visit(Expression.Constant(dateTime.Ticks));return node;case ExpressionType.MemberAccess: // Nested MemberAccess            if (node.Member.Name != ((MemberExpression)node.Expression).Member.Name) {var lambda2 = Expression.Lambda<Func<DateTime>>(node);var dateTime2 = lambda2.Compile()();base.Visit(Expression.Constant(dateTime2.Ticks));return node;}break;}}}_tokens.Add(new MemberToken(node.Expression + "." + node.Member.Name, node.Type));return node;
}protected override Expression VisitNew(NewExpression node)
{if (node.Type == typeof(DateTime)){var lambda = Expression.Lambda<Func<DateTime>>(node);var dateTime = lambda.Compile()();base.Visit(Expression.Constant(dateTime.Ticks));return node;}return base.VisitNew(node);
}

转换FilterDescriptors

public static class FilterBuilderExtensions
{public static string GetQueryString(this IList<FilterDescriptor> filters){var sb = new StringBuilder();for (var i = 0; i < filters.Count; i++){sb.Append($"filterField[{i}]={filters[i].FieldName}&" +$"filterOp[{i}]={filters[i].Operator}&" +$"filterVal[{i}]={filters[i].Value}&" +$"filterComp[{i}]={filters[i].CompositionOperator}");if (i < filters.Count - 1)sb.Append("&");}return sb.ToString();}
}

示例使用

class Program
{static void Main(string[] args){Expression<Func<Product, bool>> exp = p =>p.Id == 1009 &&!p.OutOfStock &&!(p.Price > 30000) &&!p.Name.Contains("BMW") &&p.ProductionDate > new DateTime(1999, 6, 20).Date;var visitor = new FilterBuilder(exp);var filters = visitor.Build().ToList();Console.WriteLine("Tokens");Console.WriteLine("------\n");foreach (var t in visitor.Tokens){Console.WriteLine(t);}Console.WriteLine("\nFilter Descriptors");Console.WriteLine("------------------\n");foreach (var f in filters){Console.WriteLine(f);}Console.WriteLine($"\nQuery string");Console.WriteLine("------------\n");Console.WriteLine(filters.GetQueryString());Console.ReadLine();}
}public class Product
{public int Id { get; set; }public string Name { get; set; }public decimal Price { get; set; }public DateTime ProductionDate { get; set; }public bool OutOfStock { get; set; } = false;
}

省略的功能