Issue with Multi Field/Group values and SPWorkflowTaskProperties.ExtendedProperties

I encountered an issue when assigning a multi-user data to a workflow task field from within the workfflow.

There may be various reasons anyone wants to create a User/Group field with multiple values and set it within in the workflow as below,

SPWorkflowTaskProperties.ExtendedProperties["CustomAssignToUsers"] = "domain\user1;domain\user2";

SharePoint checks the property collection and strips off the multiple user data from the property for type SPFieldUser.

As a workaround, we can create a custom field which inherits from the SPFieldUser, this new field type bypass the control type checks within SharePoint.

 using System;
 using System.Collections.Generic;
 using System.Text;
 using Microsoft.SharePoint;
 using Microsoft.SharePoint.WebControls;
 using System.Xml;
  
 namespace MyCurrentUserFieldNS
 {
     public class MyCurrentUserField : SPFieldUser
     {
         public MyCurrentUserField(SPFieldCollection fields, string fieldName)
             : base(fields, fieldName)
         {
             this.Presence = true;         
         }
         
         public override bool AllowMultipleValues
         {
             get
             {
                 return true;
             }
             set
             {
                 base.AllowMultipleValues = true;
             }
         }
  
         //There is an issue with the SPFieldUser,setting the 'AllowMultipleValues' resets the
         //Control 'Type'. As a workaround, I override this function and update the field schema
         public override void OnAdded(SPAddFieldOptions op)
         {
             base.OnAdded(op);
             XmlDocument doc = new XmlDocument();
             doc.LoadXml(this.SchemaXml);
             XmlNode element = doc.FirstChild;
             XmlAttribute attr = doc.CreateAttribute("Mult");
             attr.Value = "TRUE";
             element.Attributes.Append(attr);
             this.SchemaXml = doc.OuterXml;         
             base.Update();
  
         }        
         public MyCurrentUserField(SPFieldCollection fields, string typeName, string displayName)
             : base(fields, typeName, displayName)
         {
             this.Presence = true;
         }
         public override Type FieldValueType
         {
             get
             {
                 return typeof(SPFieldUserValueCollection);
             }
         }
         
         public override BaseFieldControl FieldRenderingControl
         {
             get
             {              
                 BaseFieldControl fldControl = new UserField();
                 fldControl.FieldName = InternalName;
                 return fldControl;             
             }
         }
     }
 }

 <FieldTypes>
     <FieldType>
         <Field Name="TypeName">MyCurrentUserField</Field>
         <Field Name="ParentType">LookupMulti</Field>
         <Field Name="TypeDisplayName">My Current User</Field>
         <Field Name="TypeShortDescription">My Current User</Field>
         <Field Name="Mult">TRUE</Field>
         <Field Name="UserCreatable">TRUE</Field>
         <Field Name="ShowInListCreate">TRUE</Field>
         <Field Name="ShowInSurveyCreate">TRUE</Field>
         <Field Name="ShowInDocumentLibraryCreate">TRUE</Field>
         <Field Name="ShowInColumnTemplateCreate">TRUE</Field>
         <Field Name="FieldTypeClass">MyCurrentUserFieldNS.MyCurrentUserField, MyCurrentUserField, Version=1.0.0.0, Culture=neutral, PublicKeyToken=79330dc34f5433ec</Field>
         <Field Name="FieldEditorUserControl">/_controltemplates/UserFieldEditor.ascx</Field>
      </FieldTypes>

Save the second markup as ‘FLDTYPES_MyCurrentUserField.xml’ and paste it under ..\12\TEMPLATE\XML folder

Build an assembly using the code snippet provided below, strong name it.

Ensure that you change the ‘FLDTYPES_MyCurrentUserField.xml’ according to the binary version and the publictokenkey.

Other Resources

Custom Field Types

https://msdn.microsoft.com/en-us/library/ms446361.aspx

Create a custom field type and a field Control

https://msdn.microsoft.com/en-us/library/bb417414.aspx

Enjoy!