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Asked: 2026-05-10T20:39:26+00:00

I want to pass an int list (List) as a declarative property to a

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I want to pass an int list (List) as a declarative property to a web user control like this:

<UC:MyControl runat='server' ModuleIds='1,2,3' />

I created a TypeConverter to do this:

public class IntListConverter : System.ComponentModel.TypeConverter {     public override bool CanConvertFrom(            System.ComponentModel.ITypeDescriptorContext context,             Type sourceType)     {         if (sourceType == typeof(string)) return true;         return base.CanConvertFrom(context, sourceType);     }     public override object ConvertFrom(       System.ComponentModel.ITypeDescriptorContext context,        System.Globalization.CultureInfo culture, object value)     {         if (value is string)         {             string[] v = ((string)value).Split(                 new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries);             List<int> list = new List<int>();             foreach (string s in vals)             {                 list.Add(Convert.ToInt32(s));             }             return list         }         return base.ConvertFrom(context, culture, value);     }     public override bool CanConvertTo(ITypeDescriptorContext context,       Type destinationType)     {         if (destinationType == typeof(InstanceDescriptor)) return true;         return base.CanConvertTo(context, destinationType);     }     public override object ConvertTo(ITypeDescriptorContext context,       System.Globalization.CultureInfo culture, object value, Type destinationType)     {         if (destinationType == typeof(InstanceDescriptor) && value is List<int>)         {             List<int> list = (List<int>)value;             ConstructorInfo construcor = typeof(List<int>).GetConstructor(new Type[] { typeof(IEnumerable<int>) });             InstanceDescriptor id = new InstanceDescriptor(construcor, new object[] { list.ToArray() });             return id;         }         return base.ConvertTo(context, culture, value, destinationType);     } }

And then added the attribute to my property:

[TypeConverter(typeof(IntListConverter))] public List<int> ModuleIds {     get { ... }; set { ... }; }

But I get this error at runtime:

Unable to generate code for a value of type 'System.Collections.Generic.List'1[[System.Int32, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'. This error occurred while trying to generate the property value for ModuleIds.

My question is similar to one found here, but the solution does not solve my problem:

Update: I found a page which solved the first problem. I updated the code above to show my fixes. The added code is the CanConvertTo and ConvertTo methods. Now I get a different error.:

Object reference not set to an instance of an object.

This error seems to be indirectly caused by something in the ConvertTo method.

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1 Answer

  1. After hooking a debugger into Cassini, I see that the null ref is actually coming from System.Web.Compilation.CodeDomUtility.GenerateExpressionForValue, which is basically trying to get an expression for the int[] array you pass into the List constructor. Since there’s no type descriptor for the int[] array, it fails (and throws a null ref in the process, instead of the ‘can’t generate property set exception’ that it should).

    I can’t figure out a built in way of getting a serializable value into a List<int>, so I just used a static method:

    class IntListConverter : TypeConverter {     public static List<int> FromString(string value) {        return new List<int>(           value            .Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries)            .Select(s => Convert.ToInt32(s))        );     }      public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType) {         if (destinationType == typeof(InstanceDescriptor)) {             List<int> list = (List<int>)value;             return new InstanceDescriptor(this.GetType().GetMethod('FromString'),                 new object[] { string.Join(',', list.Select(i => i.ToString()).ToArray()) }             );         }         return base.ConvertTo(context, culture, value, destinationType);     } }
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