问题
I've tried all sorts - lots of messing around with TypeConverters etc. So I won't cover all that here.
So to reduce the question to its basics. Considering the below:
Imports LiteDB
Imports System.ComponentModel
Public Class mSystem
<CategoryAttribute("General"), ReadOnlyAttribute(True)>
Public Property ID As Integer
Public Property ShortName As String = ""
<BsonRef("mSystemTypes")>
Public Property SystemType As mSystemType
End Class
Public Class mSystemType
<CategoryAttribute("General"), ReadOnlyAttribute(True)>
Public Property ID As Integer
Public Property Name As String = "Default"
Public Property Details As String = ""
End Class
How do I get "SystemTypes" as a dropdown selector dynamically filled from a mSystemTypes collection? E.g. You select "Console" and it updates mSystem with the matching mSystemType.
I am using LiteDb, which may make things a little more difficult, as it isn't purely an integer for the 'foreign key' as other scenarios may present, but a full object.
I need to maintain the 'dbRef' approach to ensure integrity of data relationships. Just in case the DBLite thing throws in a curve ball, some code below demonstrating its use:
Public Class dbCollecitons
Public mSystems As LiteCollection(Of mSystem)
Public mSystemTypes As LiteCollection(Of mSystemType)
Private Sub Examples()
Dim col As LiteCollection(Of mSystemType) = dbCollections.mSystemTypes
Dim value as String = "Console"
Dim q = col.FindAll.Where(Function(x) x.Name = value).First
Console.Writeline(q.ID)
End Sub
End Class
LiteDb.LiteCollection doesn't map directly onto ICollection (you use this in TypeConverter?), but I'm sure there's some work around.
回答1:
In short, you need to create a new TypeConverter supporting standard values.
Example - VB.NET
I suppose you have a Product class, having a property of type Category which you want to be able to choose from a List<Category> which is coming from somewhere like database at run-time:
Public Class Product
Public Property Id As Integer
Public Property Name As String
<TypeConverter(GetType(CategoryConverter))>
Public Property Category As Category
End Class
Public Class Category
Public Property Id As Integer
Public Property Name As String
Public Overrides Function ToString() As String
Return $"{Id} - {Name}"
End Function
End Class
And here is the CategoryService class which can load categories from wherever you like:
Public Class CategoryService
Private list As List(Of Category) = New List(Of Category) From {
New Category() With {.Id = 1, .Name = "Category 1"},
New Category() With {.Id = 2, .Name = "Category 2"},
New Category() With {.Id = 3, .Name = "Category 3"}
}
Public Function GetAll() As IEnumerable(Of Category)
Return list
End Function
End Class
The next step is creating CategoryConverter which is responsible to provide values for the dropdown:
Imports System.ComponentModel
Public Class CategoryConverter
Inherits TypeConverter
Public Overrides Function GetStandardValues(ByVal context As ITypeDescriptorContext) As StandardValuesCollection
Dim svc = New CategoryService()
Return New StandardValuesCollection(svc.GetAll().ToList())
End Function
Public Overrides Function GetStandardValuesSupported(ByVal context As ITypeDescriptorContext) As Boolean
Return True
End Function
Public Overrides Function GetStandardValuesExclusive(ByVal context As ITypeDescriptorContext) As Boolean
Return True
End Function
Public Overrides Function CanConvertFrom(ByVal context As ITypeDescriptorContext, ByVal sourceType As Type) As Boolean
If sourceType = GetType(String) Then Return True
Return MyBase.CanConvertFrom(context, sourceType)
End Function
Public Overrides Function ConvertFrom(context As ITypeDescriptorContext, culture As CultureInfo, value As Object) As Object
If value IsNot Nothing AndAlso value.[GetType]() = GetType(String) Then
Dim v = $"{value}"
Dim id = Integer.Parse(v.Split("-"c)(0).Trim())
Dim svc = New CategoryService()
Return svc.GetAll().Where(Function(x) x.Id = id).FirstOrDefault()
End If
Return MyBase.ConvertFrom(context, culture, value)
End Function
End Class
Then as a result, when you set an instance of Product as SelectedObject of the PropertyGrid, for Category property, you choose a value from list.
Example - C#
Here is the C# version of the above example:
public class Product
{
public int Id { get; set; }
public string Name { get; set; }
[TypeConverter(typeof(CategoryConverter))]
public Category Category { get; set; }
}
public class Category
{
public int Id { get; set; }
public string Name { get; set; }
public override string ToString()
{
return $"{Id} - {Name}";
}
}
public class CategoryService
{
List<Category> list = new List<Category>{
new Category() { Id = 1, Name = "Category 1" },
new Category() { Id = 2, Name = "Category 2" },
new Category() { Id = 3, Name = "Category 3" },
};
public IEnumerable<Category> GetAll()
{
return list;
}
}
public class CategoryConverter : TypeConverter
{
public override StandardValuesCollection GetStandardValues(ITypeDescriptorContext context)
{
var svc = new CategoryService();
return new StandardValuesCollection(svc.GetAll().ToList());
}
public override bool GetStandardValuesSupported(ITypeDescriptorContext context)
{
return true;
}
public override bool GetStandardValuesExclusive(ITypeDescriptorContext context)
{
return true;
}
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
if (sourceType == typeof(string))
return true;
return base.CanConvertFrom(context, sourceType);
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
if (value != null && value.GetType() == typeof(string))
{
var v = $"{value}";
var id = int.Parse(v.Split('-')[0].Trim());
var svc = new CategoryService();
return svc.GetAll().Where(x => x.Id == id).FirstOrDefault();
}
return base.ConvertFrom(context, culture, value);
}
}
来源：https://stackoverflow.com/questions/53566675/propertygrid-load-dropdown-values-dynamically