Please refer to this code
public class A : B
{
[Display(Name = "Initial Score Code", Order =3)]
public Code { get; set; }
[Display(Name = "Initial Score Code", Order =2)]
public Name{ get; set; }
............
}
I need to get all properties of class through order by orderAttribute of Display. I have tried with this code to do
var prop = typeof(A)
.GetProperties()
.OrderBy(p => ((DisplayAttribute)p.GetCustomAttributes(typeof(DisplayAttribute), true).FirstOrDefault).Order);
but it causes an error
object reference not to set an instance of object
I assumed this issue because of some property not having "Order" property in "DisplayAttribute" .
How to handle this kind of situation? I need to order all the properties even though some property not having the value of order property.
You are missing brackets ()
on FirstOrDefault
operator. Also you should deal with case when default value is returned. I suggest to select Order
value before getting first or default value. That will return 0
for all properties which don't have DisplayAttribute
:
var prop = typeof(A)
.GetProperties()
.OrderBy(p => p.GetCustomAttributes(typeof(DisplayAttribute), true)
.Cast<DisplayAttribute>()
.Select(a => a.Order)
.FirstOrDefault());
If you want properties without DisplayAttribute to be last, you can provide Int32.MaxValue
as default value to be returned:
.Select(a => a.Order)
.DefaultIfEmpty(Int32.MaxValue)
.First()
Try this:
var props = from p in typeof(A).GetProperties()
let orderAttribute = (DisplayAttribute)(p.GetCustomAttributes(typeof(DisplayAttribute), true))
.FirstOrDefault()
where orderAttribute != null
orderby orderAttribute.Order
select p;
Or:
var props = from p in typeof(A).GetProperties()
let orderAttribute = (DisplayAttribute)(p.GetCustomAttributes(typeof(DisplayAttribute), true))
.FirstOrDefault()
orderby orderAttribute == null ? 0 : orderAttribute.Order
select p;
Here is a much more complete answer that allows you to better control the ordering of PropertyInfo
instances without DisplayAttribute
attributes:
public class A
{
[Display(Name = "Initial Score Code", Order = 3)]
public int Code
{
get;
set;
}
[Display(Name = "Initial Score Code", Order = 2)]
public string Name
{
get;
set;
}
}
public class PropertyInfoComparer : IComparer<PropertyInfo>
{
public int Compare(PropertyInfo x, PropertyInfo y)
{
var attribute1 = (DisplayAttribute)x.GetCustomAttributes(typeof(DisplayAttribute)).FirstOrDefault();
var attribute2 = (DisplayAttribute)y.GetCustomAttributes(typeof(DisplayAttribute)).FirstOrDefault();
// If the first property has no attribute, sort it first
if (attribute1 == null)
{
return -1;
}
// If the second property has no attribute, sort it first
if (attribute2 == null)
{
return 1;
}
// Compare the Order properties of both attributes
return attribute1.Order.CompareTo(attribute2.Order);
}
}
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class DisplayAttribute : Attribute
{
public string Name
{
get;
set;
}
public int Order
{
get;
set;
}
}
Usage:
// Get all the properties of Foo and order them using PropertyInfoComparer
typeof(Foo).GetProperties().OrderBy(arg => arg, new PropertyInfoComparer());
I like the approach with Comparer. However, when I tried it, my iterator went into a dead loop at first. Later, it started throwing exceptions. Also, I optimized it for the case when first property doesn't contain "Order" descriptor to avoid even checking for a second one. I also moved all comments into the class description:
/// <summary>
/// If the first property has no attribute, sort it first
/// If the second property has no attribute, sort it first
/// Compare the Order properties of both attributes
/// </summary>
public class PropertyInfoComparer : IComparer<PropertyInfo>
{
public int Compare(PropertyInfo x, PropertyInfo y)
{
if (x == y) return 0;
var attrX = (DisplayAttribute)x.GetCustomAttributes(typeof(DisplayAttribute)).FirstOrDefault();
int? orderX = attrX?.GetOrder();
if (orderX == null) return -1;
var attrY = (DisplayAttribute)y.GetCustomAttributes(typeof(DisplayAttribute)).FirstOrDefault();
int? orderY = attrY?.GetOrder();
if (orderY == null) return 1;
return ((int)orderX).CompareTo((int)orderY);
}
}
Unfortunately, classes that have properties without the "Order" descriptor lose their "natural" order. Therefore, I ended up checking for any properties that have the "Order" descriptor first. If at least one of them has that descriptor, then do the sorting.
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