动态构建Lambda

[专业各种打杂]

[圣殿骑士18]

以前写过类似的一个功能，在EF保存前去除Modified状态的少数几个字段。

context.Entry(item).State = System.Data.Entity.EntityState.Modified;
                    context.RemoveModifiedStatus(item, c => new { c.CheckUserId, c.CheckTime, c.PublishTime, c.Guid });

[圣殿骑士18]

实现的方法，主要是解析表达式树，写的时候对表达式树不是很熟，实现方式走了弯路，不过能用就没改了，可参考：

public static void RemoveModifiedStatus<TSource>(this DbContext context, TSource model, Expression<Func<TSource, object>> keySelector)
            where TSource : class
        {
            //取得表达式
            var expression = (new List<TSource>())
                .AsQueryable()
                .OrderBy(keySelector)
                .Expression;

            //取得表达式定义
            //root.Nodes[1].Nodes[0].Nodes[0].Nodes
            ExpressionTreeNode root = ExpressionTreeBuilder.GetExpressionTreeNode(expression);
            if (root == null || root.Nodes[1] == null || root.Nodes[1].Nodes[0] == null || root.Nodes[1].Nodes[0].Nodes[0] == null || root.Nodes[1].Nodes[0].Nod

这个方法来依赖一个三方类库ExpressionTreeBuilder，很麻烦。

[圣殿骑士18]

实际上，解析keySelector不用这么复杂，直接解析其Body属性就行，参考如下方法，调试一下，从Body中解析出你要的东西，即可：

public static string GetPropName<T>(Expression<Func<T, object>> expr)
        {
            switch (expr.Body.NodeType)
            {
                case ExpressionType.MemberAccess:
                    return ((MemberExpression)expr.Body).Member.Name;
                case ExpressionType.Convert:
                    return ((MemberExpression)((UnaryExpression)expr.Body).Operand).Member.Name;
                case ExpressionType.New:
                    //new {} 的 解析在这里
                    return null;
                default:
                    return null;
            }
        }

[xuzuning]

借助 json 或 xml，应该很好实现的

[以专业开发人员为伍]

csdn的图片不再能点击查看原图了，对于缩小了的图片懒得看！贴你的源代码。（csdn对于源代码的显示也变成垃圾的黑体了，失去了源代码的样式。csdn 越来越充斥这混乱的颜色、图标、不重要的文本列）

[threenewbee]

构建不了，这个不是linq表达式里构建的，而是C#编译器在编译期间（注意，是编译期间，而不是运行时）产生的匿名类型。

[mjzxlmg]

引用 楼主 专业各种打杂 的回复:

#Region "动态构建Selector表达式"
#Region "BuildSelectorExpression"
    ''' <summary>
    ''' 动态构建Selector表达式返回查询表达式
    ''' </summary>
    ''' <typeparam name="T"></typeparam>
    ''' <param name="sources"></param>
    ''' <param name="fields"></param>
    ''' <returns></returns>
    <Runtime.CompilerServices.Extension>
    Public Function SelectDynamic(Of T)(sources As IEnumerable(Of T), fields As IEnumerable(Of String)) As IEnumerable(Of Object)
        Return sources.AsQueryable.Select(BuildSelectorExpression(Of T)(fields))
    End Function
    ''' <summary>
    ''' 构建Selector表达式
    ''' </summary>
    ''' <param name="fields">需要选择的属性名称</param>
    ''' <returns>Selector表达式</returns>
    <Runtime.CompilerServices.Extension>
    Public Function BuildSelectorExpression(Of T)(fields As IEnumerable(Of String)) As Expression(Of Func(Of T, Object))
        Dim Properties_dic As New Dictionary(Of String, PropertyInfo)(GetType(T).GetRuntimeProperties.Where(Function(p) fields.Contains(p.Name)).ToDictionary(Function(p) p.Name, Function(p) p))
        Dim DynamicType As Type = LinqRuntimeTypeBuilder.GetDynamicType(Properties_dic.Values)
        Dim Parameter As ParameterExpression = Expression.Parameter(GetType(T), "p")
        Dim MemberBindings As IEnumerable(Of MemberBinding) = DynamicType.GetRuntimeProperties().Select(Function(p) Expression.Bind(p, Expression.Property(Parameter, Properties_dic(p.Name)))).OfType(Of MemberBinding)()
        Return Expression.Lambda(Of Func(Of T, Object))(Expression.MemberInit(Expression.[New](DynamicType.GetConstructor(Type.EmptyTypes)), MemberBindings), Parameter)
    End Function
#End Region
#Region "LinqRuntimeTypeBuilder"
    Public NotInheritable Class LinqRuntimeTypeBuilder
        Private Shared ReadOnly AssemblyName As New AssemblyName() With {.Name = "LinqRuntimeTypes"}
        Private Shared ReadOnly ModuleBuilder As ModuleBuilder
        Private Shared ReadOnly BuiltTypes_dic As New Dictionary(Of String, Type)()
        Private Const RuntimeGetSetAttrs As MethodAttributes = MethodAttributes.Public Or MethodAttributes.SpecialName Or MethodAttributes.HideBySig
        Shared Sub New()
            ModuleBuilder = AssemblyBuilder.DefineDynamicAssembly(AssemblyName, AssemblyBuilderAccess.Run).DefineDynamicModule(AssemblyName.Name)
        End Sub

        ''' <summary>
        ''' BuildDynamicType
        ''' </summary>
        ''' <param name="properties_dic"></param>
        ''' <returns></returns>
        Public Shared Function BuildDynamicType(properties_dic As Dictionary(Of String, Type)) As Type
            If properties_dic Is Nothing Then Throw New ArgumentNullException(NameOf(properties_dic))
            If properties_dic.Count = 0 Then Throw New ArgumentOutOfRangeException(NameOf(properties_dic), "必须至少一个字段")
            Try
                Monitor.Enter(BuiltTypes_dic)
                Dim className As String = GetTypeKey(properties_dic)
                If BuiltTypes_dic.ContainsKey(className) Then Return BuiltTypes_dic(className)
                Dim TypeBuilder As TypeBuilder = ModuleBuilder.DefineType(className, TypeAttributes.Public Or TypeAttributes.Class Or TypeAttributes.Serializable)
                With TypeBuilder
                    For Each prop As KeyValuePair(Of String, Type) In properties_dic
                        Dim PropertyBuilder As PropertyBuilder = .DefineProperty(name:=prop.Key, attributes:=PropertyAttributes.None, returnType:=prop.Value, parameterTypes:=Nothing)
                        Dim FieldBuilder As FieldBuilder = .DefineField("itheofield_" & prop.Key, prop.Value, FieldAttributes.Private)
                        Dim getMethodBuilder As MethodBuilder = .DefineMethod("get_" & prop.Key, RuntimeGetSetAttrs, prop.Value, Type.EmptyTypes)
                        Dim GetILGenerator As ILGenerator = getMethodBuilder.GetILGenerator()
                        With GetILGenerator
                            .Emit(OpCodes.Ldarg_0)
                            .Emit(OpCodes.Ldfld, FieldBuilder)
                            .Emit(OpCodes.Ret)
                        End With
                        Dim setMethodBuilder As MethodBuilder = TypeBuilder.DefineMethod("set_" & prop.Key, RuntimeGetSetAttrs, Nothing, New Type() {prop.Value})
                        Dim setILGenerator As ILGenerator = setMethodBuilder.GetILGenerator()
                        With setILGenerator
                            .Emit(OpCodes.Ldarg_0)
                            .Emit(OpCodes.Ldarg_1)
                            .Emit(OpCodes.Stfld, FieldBuilder)
                            .Emit(OpCodes.Ret)
                        End With
                        PropertyBuilder.SetGetMethod(getMethodBuilder)
                        PropertyBuilder.SetSetMethod(setMethodBuilder)
                    Next
                    BuiltTypes_dic(className) = .CreateType()
                    Return BuiltTypes_dic(className)
                End With
            Catch
                Throw
            Finally
                Monitor.Exit(BuiltTypes_dic)
            End Try
        End Function

        ''' <summary>
        ''' GetTypeKey
        ''' </summary>
        ''' <param name="properties_dic"></param>
        ''' <returns></returns>
        Private Shared Function GetTypeKey(properties_dic As Dictionary(Of String, Type)) As String
            Return String.Join(";", properties_dic.Select(Function(p) p.Key & ";" & p.Value.Name).ToArray)
        End Function

        ''' <summary>
        ''' GetDynamicType
        ''' </summary>
        ''' <param name="properties"></param>
        ''' <returns></returns>
        Public Shared Function GetDynamicType(properties As IEnumerable(Of PropertyInfo)) As Type
            Return BuildDynamicType(New Dictionary(Of String, Type)(properties.ToDictionary(Function(p) p.Name, Function(p) p.PropertyType)))
        End Function
    End Class
#End Region
#End Region