LINQ 中的动态继承映射答案

【问题标题】：Dynamic Inheritance Mapping in LINQLINQ 中的动态继承映射
【发布时间】：2013-01-27 05:21:29
【问题描述】：

当我知道类型：客户和员工时，这是一个继承映射。

modelBuilder.Entity<Person>()
            .Map<Customer>(c => c.Requires("Type").HasValue("Customer"))
            .Map<Employee>(e => e.Requires("Type").HasValue("Employee"))
            .ToTable("People", "dbo");

在我的例子中，Customer 和 Employee 类将驻留在插件程序集中。所以，我正在尝试使用反射创建映射。

注意： 为了使示例简单，我对类型进行了硬编码。另外，我只是想为 Customer 类型创建映射。

const BindingFlags binding = BindingFlags.Public | BindingFlags.Instance;

EntityTypeConfiguration<Person> entityConfig = modelBuilder.Entity<Person>();
var entityConfigType = entityConfig.GetType();

// Map Method
MethodInfo mapMethod = null;
var methods = entityConfigType.GetMethods(binding);
foreach (var method in methods)
{
    if (method.Name == "Map" && method.IsGenericMethod)
    {
        mapMethod = method.MakeGenericMethod(typeof(Customer));
        break;
    }
}

// Requires Method
Type[] requiresArgType = new Type[]
    {
        typeof(string)
    };

var mappingConfigType = typeof(EntityMappingConfiguration<Customer>);
var requiresMethod = mappingConfigType.GetMethod("Requires", binding, null, requiresArgType, null);

// Has Value Method
var vccType = typeof (ValueConditionConfiguration);
Type[] hasValueArgType = new Type[]
    {
        typeof(string)
    };

var hasValueMethod = vccType.GetMethod("HasValue", binding, null, hasValueArgType, null);

var param1 = Expression.Parameter(typeof (EntityMappingConfiguration<Customer>), "c");
var requiresCall1 = Expression.Call(param1, requiresMethod, Expression.Constant("Type"));
var hasValueCall1 = Expression.Call(requiresCall1, hasValueMethod, Expression.Constant("Customer"));

var lambda1 = Expression.Lambda(hasValueCall1, param1);
var @delegate1 = lambda1.Compile();

mapMethod.Invoke(entityConfig, new object[]
    {
        @delegate1
    });

当试图调用 Map 方法时，抛出以下异常：-

Object of type 'System.Func`2[System.Data.Entity.ModelConfiguration.Configuration.EntityMappingConfiguration`1[ConsoleApplication1.Customer],System.Data.Entity.ModelConfiguration.Configuration.StringColumnConfiguration]' cannot be converted to type 'System.Action`1[System.Data.Entity.ModelConfiguration.Configuration.EntityMappingConfiguration`1[ConsoleApplication1.Customer]]'.

如何转换

Func<EntityMappingConfiguration<Customer>> with return type of StringColumnConfiguration

到

Action<EntityMappingConfiguration<Customer>>

【问题讨论】：

Lambda 方法的通用变体成功了。
您好，您介意在回答您自己的问题时详细说明这一点，然后将其标记为已接受吗？（别担心，这是 StackOverflow 的常见做法）。

标签： c# linq-to-entities

【解决方案1】：

我换了

var lambda1 = Expression.Lambda(hasValueCall1, param1);

以下

MethodInfo[] expressionMethods = typeof (Expression).GetMethods(BindingFlags.Public | BindingFlags.Static);

MethodInfo lambdaMethod = null;
foreach (MethodInfo method in expressionMethods)
{
    if (method.Name == "Lambda" && method.IsGenericMethod && method.GetParameters().Length == 2)
    {
        lambdaMethod = method;
        break;
    }
}

MethodInfo genericLambdaMethod = lambdaMethod.MakeGenericMethod(typeof (Action<>).MakeGenericType(typeof<EntityMappingConfiguration<Customer>>));

LambdaExpression lambda1 = (LambdaExpression) genericLambdaMethod.Invoke(null, new object[]
{
    hasValueCall1, new[]
    {
        param1
    }
});

【讨论】：