来自 sql 查询执行实体框架的匿名类型结果 [英] Anonymous type result from sql query execution entity framework

问题描述

我正在使用实体框架 5.0 和 .net 框架 4.0 代码优先方法.现在我知道我可以按照

I am using entity framework 5.0 with .net framework 4.0 code first approach. Now i know that i can run raw sql in entity framework by following

var students = Context.Database.SqlQuery<Student>("select * from student").ToList();

它运行良好，但我想要的是返回匿名结果.例如，我只想要学生表中的特定列，如下所示

It's working perfectly but what I want is return anonymous results. For example I want only specific columns from student table like following

var students = Context.Database.SqlQuery<Student>("select FirstName from student").ToList();

它不起作用.它给出了例外

It is not working. it gives exception

数据读取器与指定的MyApp.DataContext.Student"不兼容.'StudentId' 类型的成员在数据读取器中没有对应的同名列.

The data reader is incompatible with the specified 'MyApp.DataContext.Student'. A member of the type, 'StudentId', does not have a corresponding column in the data reader with the same name.

所以我尝试了 dynamic 类型

So I have tried dynamic type

var students = Context.Database.SqlQuery<dynamic>("select FirstName from student").ToList();

它也不起作用，它返回一个空对象.其中没有可用数据.

it is also not working, it returns an empty object. No data available in it.

有没有办法从动态 SQL 查询中获取匿名类型的结果?

Is there any way to get anonymous type result from a dynamic SQL query?

推荐答案

这是对我来说效果很好的最终解决方案.

Here is final solution that worked fine for me.

public static System.Collections.IEnumerable DynamicSqlQuery(this Database database, string sql, params object[] parameters)
        {
            TypeBuilder builder = createTypeBuilder(
                    "MyDynamicAssembly", "MyDynamicModule", "MyDynamicType");

            using (System.Data.IDbCommand command = database.Connection.CreateCommand())
            {
                try
                {
                    database.Connection.Open();
                    command.CommandText = sql;
                    command.CommandTimeout = command.Connection.ConnectionTimeout;
                    foreach (var param in parameters)
                    {
                        command.Parameters.Add(param);
                    }

                    using (System.Data.IDataReader reader = command.ExecuteReader())
                    {
                        var schema = reader.GetSchemaTable();

                        foreach (System.Data.DataRow row in schema.Rows)
                        {
                            string name = (string)row["ColumnName"];
                            //var a=row.ItemArray.Select(d=>d.)
                            Type type = (Type)row["DataType"];
                            if(type!=typeof(string) && (bool)row.ItemArray[schema.Columns.IndexOf("AllowDbNull")])
                            {
                                type = typeof(Nullable<>).MakeGenericType(type);
                            }
                            createAutoImplementedProperty(builder, name, type);
                        }
                    }
                }
                finally
                {
                    database.Connection.Close();
                    command.Parameters.Clear();
                }
            }

            Type resultType = builder.CreateType();

            return database.SqlQuery(resultType, sql, parameters);
        }

        private static TypeBuilder createTypeBuilder(
            string assemblyName, string moduleName, string typeName)
        {
            TypeBuilder typeBuilder = AppDomain
                .CurrentDomain
                .DefineDynamicAssembly(new AssemblyName(assemblyName),
                                       AssemblyBuilderAccess.Run)
                .DefineDynamicModule(moduleName)
                .DefineType(typeName, TypeAttributes.Public);
            typeBuilder.DefineDefaultConstructor(MethodAttributes.Public);
            return typeBuilder;
        }

        private static void createAutoImplementedProperty(
            TypeBuilder builder, string propertyName, Type propertyType)
        {
            const string PrivateFieldPrefix = "m_";
            const string GetterPrefix = "get_";
            const string SetterPrefix = "set_";

            // Generate the field.
            FieldBuilder fieldBuilder = builder.DefineField(
                string.Concat(PrivateFieldPrefix, propertyName),
                              propertyType, FieldAttributes.Private);

            // Generate the property
            PropertyBuilder propertyBuilder = builder.DefineProperty(
                propertyName, System.Reflection.PropertyAttributes.HasDefault, propertyType, null);

            // Property getter and setter attributes.
            MethodAttributes propertyMethodAttributes =
                MethodAttributes.Public | MethodAttributes.SpecialName |
                MethodAttributes.HideBySig;

            // Define the getter method.
            MethodBuilder getterMethod = builder.DefineMethod(
                string.Concat(GetterPrefix, propertyName),
                propertyMethodAttributes, propertyType, Type.EmptyTypes);

            // Emit the IL code.
            // ldarg.0
            // ldfld,_field
            // ret
            ILGenerator getterILCode = getterMethod.GetILGenerator();
            getterILCode.Emit(OpCodes.Ldarg_0);
            getterILCode.Emit(OpCodes.Ldfld, fieldBuilder);
            getterILCode.Emit(OpCodes.Ret);

            // Define the setter method.
            MethodBuilder setterMethod = builder.DefineMethod(
                string.Concat(SetterPrefix, propertyName),
                propertyMethodAttributes, null, new Type[] { propertyType });

            // Emit the IL code.
            // ldarg.0
            // ldarg.1
            // stfld,_field
            // ret
            ILGenerator setterILCode = setterMethod.GetILGenerator();
            setterILCode.Emit(OpCodes.Ldarg_0);
            setterILCode.Emit(OpCodes.Ldarg_1);
            setterILCode.Emit(OpCodes.Stfld, fieldBuilder);
            setterILCode.Emit(OpCodes.Ret);

            propertyBuilder.SetGetMethod(getterMethod);
            propertyBuilder.SetSetMethod(setterMethod);
        }    

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