从参数中的查询返回查询的方法

Question

演示文稿

我有一个带有一些属性的ContactProfileModel 实体类：

• 名字
• 姓氏
• 出生日期等。

我还有其他拥有ContactProfileModel 外键的实体。示例：RegistrationModel.Contact.

需要

我想创建一个带有以下签名的方法：

public static Expression<Func<TModel, string>> Contact<TModel>(Expression<Func<TModel, ContactProfileModel>> contact)

并以这种方式使用它：

DisplayQuery.Contact<RegistrationModel>(m => m.ContactProfile))

相当于

m => m.ContactProfile.FirstName + " " + m.ContactProfile.FirstName + " " + m.ContactProfile.BirthDate.ToShortTimeString()

目标

目标是返回一个 linq 查询，其中 result 是一个字符串，包含联系人的不同信息。示例：“John Doe (10/10/90)”

注意

我与一些人讨论过，他们告诉我使用Expression.Call 和Expression.Property，但不幸的是我没有足够的知识来正确使用它。

在这里我没有额外的细节就暴露了我的问题，但我有理由只以这种方式创建我的方法。

提前致谢。

Answer 1

这是一个完整的工作实现：代码运行并输出您所期望的。

我的时间有点短，所以我就这样吧。如果您想澄清，请在 cmets 中询问，我会尽力回答。

public class Program
{
    private static readonly MethodInfo stringConcatMethod = typeof(string).GetMethod("Concat", new[] { typeof(string[]) });
    private static readonly MethodInfo toShortTimeStringMethod = typeof(DateTime).GetMethod("ToShortTimeString");
    private static readonly PropertyInfo firstNameProperty = typeof(ContactProfileModel).GetProperty("FirstName");
    private static readonly PropertyInfo lastNameProperty = typeof(ContactProfileModel).GetProperty("LastName");
    private static readonly PropertyInfo birthDateProperty = typeof(ContactProfileModel).GetProperty("BirthDate");

    public static void Main()
    {
        var result = Contact<RegistrationModel>(x => x.ContactProfile);

        // Test it
        var model = new RegistrationModel()
        {
            ContactProfile = new ContactProfileModel()
            {
                FirstName = "First",
                LastName = "Last",
                BirthDate = DateTime.Now,
            }
        };
        var str = result.Compile()(model);
    }

    public static Expression<Func<TModel, string>> Contact<TModel>(Expression<Func<TModel, ContactProfileModel>> contact)
    {
        // We've been given a LambdaExpression. It's got a single
        // parameter, which is the 'x' above, and its body
        // should be a MemberExpression accessing a property on
        // 'x' (you might want to check this and throw a suitable
        // exception if this isn't the case). We'll grab the
        // body of the LambdaExpression, and use that as the
        // 'm.ContactProfile' expression in your question. 
        // At the end, we'll construct a new LambdaExpression
        // with our body. We need to use the same ParameterExpression
        // given in this LambdaExpression.
        var modelParameter = contact.Parameters[0];
        var propertyAccess = (MemberExpression)contact.Body;

        // <contact>.FirstName
        var firstNameAccess = Expression.Property(propertyAccess, firstNameProperty);
        // <contact>.LastName
        var lastNameAccess = Expression.Property(propertyAccess, lastNameProperty);
        // <contact>.BirthDate
        var birthDateAccess = Expression.Property(propertyAccess, birthDateProperty);
        // <contact>.BirthDate.ToShortTimeString()
        var birthDateShortTimeStringCall = Expression.Call(birthDateAccess, toShortTimeStringMethod);

        // string.Concat(new string[] { <contact>.FirstName, " ", etc }
        var argsArray = Expression.NewArrayInit(typeof(string), new Expression[]
        {
            firstNameAccess,
            Expression.Constant(" "),
            lastNameAccess,
            Expression.Constant(" "),
            birthDateShortTimeStringCall
        });
        var concatCall = Expression.Call(stringConcatMethod, argsArray);

        // Turn it all into a LambdaExpression
        var result = Expression.Lambda<Func<TModel, string>>(concatCall, modelParameter);
        // Note: if you inspect 'result.DebugView' in a debugger at this 
        // point, you'll see a representation of the expression we've built
        // up above, which is useful for figuring out where things have gone
        // wrong.
        return result;
    }
}

public class ContactProfileModel
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime BirthDate { get; set; }
}

public class RegistrationModel
{
    public ContactProfileModel ContactProfile { get; set; }
}

可能是 EF 不喜欢调用 String.Concat - 在这种情况下，您可能不得不使用一组 Expression.Add 来代替。

Answer 2

StackOverflow 上的第一个答案，请善待 ;)

我试图解决这个问题，但表达式并不容易使用。感谢canton7的回答。如果您想在表达式中使用 .ToString() 方法，我编辑了我的答案以显示解决方案。

public class ContactProfileModel
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime BirthDate { get; set; }

    public override string ToString()
    {
        return $"{FirstName} {LastName} {BirthDate.ToShortDateString()}";
    }
}

public class RegistrationModel
{
    public ContactProfileModel ContactProfile { get; set; }
}

public class Program
{
    static void Main(string[] args)
    {
        var registration = new RegistrationModel
        {
            ContactProfile = new ContactProfileModel
            {
                FirstName = "John",
                LastName = "Doe",
                BirthDate = DateTime.Now
            }
        };

        var expression = Contact<RegistrationModel>(m => m.ContactProfile);

        Console.WriteLine(expression.Compile()(registration));
        Console.ReadKey();
    }

    public static Expression<Func<TModel, string>> Contact<TModel>(Expression<Func<TModel, ContactProfileModel>> contact)
    {
        var propertyAccess = (MemberExpression)contact.Body;
        var toString = typeof(ContactProfileModel).GetMethod("ToString");
        var toStringValue = Expression.Call(propertyAccess, toString);

        return Expression.Lambda<Func<TModel, string>>(toStringValue, contact.Parameters[0]);
    }
}