如何以编程方式更新 DisplayAttribute 上的 Order 属性？

Question

Given the following code:

using System.ComponentModel.DataAnnotations;
using System.Linq;
using Xunit;

namespace Company.Tests
{
    public class MyObject
    {
        [Display(Order = 1000)]
        public virtual string StringPropertyB { get; set; }

        [Display(Order = 2000)]
        public virtual string StringPropertyA { get; set; }
    }

    public class MyObjectTest
    {
        [Fact]
        public void X()
        {
            var properties = typeof(MyObject).GetProperties();
            var stringPropertyBPropertyInfo = properties[0];
            var stringPropertyAPropertyInfo = properties[1];

            // Debugger Display = "{[System.ComponentModel.DataAnnotations.DisplayAttribute(Order = 1000)]}"
            var bDisplayAttribute = stringPropertyBPropertyInfo.GetCustomAttributesData().FirstOrDefault();

            // Debugger Display = "{[System.ComponentModel.DataAnnotations.DisplayAttribute(Order = 2000)]}"
            var aDisplayAttribute = stringPropertyAPropertyInfo.GetCustomAttributesData().FirstOrDefault();
        }
    }
}

How do I update the Order property programmatically?

I want to update it after it's been set to a new value. (Use case being not having to specify the Order but instead assign the Order a value automatically to match the order that the properties on MyObject appear from top to bottom.)

Answer 1

Just as Mr Skeet wrote in the answer that Thumper pointed to this can't be done as far as I can tell. You can update the Order value but it won't persist:

using System.ComponentModel.DataAnnotations;
using System.Linq;
using System.Reflection;
using Shouldly;
using Xunit;

namespace Tests
{
    public class ObjectWithDisplayOrder
    {
        [Display(Order = 0)]
        public virtual string StringPropertyB { get; set; }

        [Display(Order = 0)]
        public virtual string StringPropertyA { get; set; }
    }

    public class DisplayOrderTests
    {
        [Fact]
        public void ShouldUpdateDisplayOrderProperty()
        {
            const int updatedOrderValue = 1000;

            var properties = typeof(ObjectWithDisplayOrder).GetProperties();
            foreach (var property in properties)
            {
                var displayAttribute = (DisplayAttribute) property.GetCustomAttributes().First(a => a is DisplayAttribute);
                var props = displayAttribute.GetType().GetProperties(BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public).ToList();
                props.Single(p => p.Name == "Order").SetValue(displayAttribute, updatedOrderValue);
                // displayAttribute Order is 1000 here, but it's not persisted...
            }

            foreach (var property in properties)
            {
                var displayAttribute = (DisplayAttribute) property.GetCustomAttributes().First(a => a is DisplayAttribute);
                displayAttribute.GetOrder().ShouldBe(updatedOrderValue); // Fails - Order is still 0
            }
        }
    }
}

Answer 2

Instead of what you have do this

public class MyObject
{
    [DefaultValue(1000)]
    public virtual int StringPropertyBOrder { get; set; }
    public virtual string StringPropertyB { get; set; }

    [DefaultValue(2000)]
    public virtual int StringPropertyAOrder { get; set; }
    public virtual string StringPropertyA { get; set; }
}

Answer 3

Updated Answer: Because attributes are "static metadata", they cannot be persistently changed after compilation. ( Can attributes be added dynamically in C#? )

So, one way to accomplish OP's request to make persistent changes to Attribute values is to compile the code at runtime . If this is what's needed, it may be worth seriously considering a different approach to the problem you are trying to solve, since this means a lot more work for you (but it's super cool to me).

There is also the possiblity of using something called TypeBuilder ( How Can I add properties to a class on runtime in C#? ). I am not familiar with this, but it looks promising. It does also look like it uses runtime compilation.

Runtime Compiling:

Here is one way this can be accomplished, by compiling code at runtime. (Using NUnit, not XUnit, as OP has done):

namespace OpUnitTest {

    [TestClass]
    public class OpTest{

        //Use some web templating model so we can easily change it later (#=variable#)
        string myClassToCompile = @"

using System.ComponentModel.DataAnnotations;

namespace Test {
    public class ObjectWithDisplayOrder {
        [Display(Order = #=0#)]
        public virtual string StringPropertyB { get; set; }

        [Display(Order = #=1#)]
        public virtual string StringPropertyA { get; set; }
    }
}
";
        [TestMethod]
        public void AssignAtributeValuesDynamically() {

            const int order = 1000;     

            //Escape curly braces.
            myClassToCompile = myClassToCompile.Replace("{", "{{").Replace("}", "}}");

            //We could use Regex, or even a for loop, to make this more-elegant and scalable, but this is a Proof of Concept.
            myClassToCompile = myClassToCompile.Replace("#=0#", "{0}").Replace("#=1#", "{1}");

            myClassToCompile = string.Format(myClassToCompile, order, order);

            CSharpCodeProvider provider = new CSharpCodeProvider();
            CompilerParameters parameters = new CompilerParameters();
            parameters.ReferencedAssemblies.Add("System.ComponentModel.DataAnnotations.dll");
            parameters.GenerateInMemory = true;
            CompilerResults results = provider.CompileAssemblyFromSource(parameters, myClassToCompile);

            //You would normally check for compilation errors, here.

            Assembly assembly = results.CompiledAssembly;
            Type myCompiledObject = assembly.GetType("Test.ObjectWithDisplayOrder");
            PropertyInfo[] properties = myCompiledObject.GetProperties();

            foreach (var property in properties) {
                var displayAttribute = (DisplayAttribute)property.GetCustomAttributes().First(a => a is DisplayAttribute);
                Assert.AreEqual(order, displayAttribute.GetOrder());

            }
        }
}

A good ramp-up guide on runtime compiling in C# (something that is a bit of a passion of mine): http://www.codeproject.com/Tips/715891/Compiling-Csharp-Code-at-Runtime

Original Answer:

Technically, it demonstrates that you can, indeed, change the attribute's value, but as OP pointed out - this doesn't persist.

using System.ComponentModel.DataAnnotations;
using System.Linq;
using Xunit;

namespace Company.Tests
{
    public class MyObject
    {
        [Display(Order = 1000)]
        public virtual string StringPropertyB { get; set; }

        [Display(Order = 2000)]
        public virtual string StringPropertyA { get; set; }
    }

    public class MyObjectTest
    {
        [Fact]
        public void X()
        {
            var properties = typeof(MyObject).GetProperties();
            var stringPropertyBPropertyInfo = properties[0];
            var bDisplayAttribute = (DisplayAttribute)stringPropertyBPropertyInfo.GetCustomAttributes().First();
            var props = bDisplayAttribute.GetProperties(BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public).ToList();
            props.Single(p => p.Name == "Order").SetValue(bDisplayAttribute, 5);

        }
    }
}