System.Data.DataColumn serializes the DataType property with type byte[] incorrectly

[rmkarasev]

Environment

Microsoft Visual Studio 2026
Version: 18.2.0

.NET version

.NET 8

Did this work in a previous version of Visual Studio and/or previous .NET release?

Works on .NET Framework

Issue description

We have a custom Component and ComponentDesigner to set up data sources at the design time on Forms and UserControls.
After migration to .NET 8, we faced an issue where DataColumn serializes byte[] type in its DataType property incorrectly. Designer saves it as byte.

The DataColumn.DataType Property doc states that byte array is still an allowed type. Also I found no breaking changes related to this property and believe that obsolescence of BinaryFormatter has no bearing here.
For example, we use this type for documents/images/other attachments on multiple forms.

After any form change, the designer replaces DataType typeof(byte[]) -> typeof(byte), and source code becomes incompatible with data provided from the storage.

Previous code (.NET Framework)

this.dataColumn5.ColumnName = "Data";
this.dataColumn5.DataType = typeof(byte[]);

Updated code (.NET)

dataColumn5.ColumnName = "Data";
dataColumn5.DataType = typeof(byte);

Any feedback would be appreciated, as we are stuck on updating our forms.

Steps to reproduce

Minimal project is attached. It's different from our component, but should show the same issue.
Form1.Designer.cs contains typeof(byte), but actual type should be typeof(byte[])

DataColumnByteArray.zip

[merriemcgaw]

@KlausLoeffelmann to take a look.
@Olina-Zhang can you help us understand if/when this regressed in the OOP designer? It shouldn't be too rare of a scenario so we're surprised to see it pop up now.

[Olina-Zhang]

@merriemcgaw this issue occurs .NET designer involved modernized initialization.

Repro steps:

1. Create a WinForms .NET project with button control added
2. In Form1.cs Class, add private System.Data.DataColumn column;
3. In InitializeComponent() of Form1.Designer.cs file, add these codes:

column = new DataColumn();
column.DataType = typeof(byte[]);

4. Build and resize form designer
5. Open Form1.designer.cs file, observe the codes we added

Result: it changes to column.DataType = typeof(byte);

[Olina-Zhang]

@merriemcgaw can you check if it is the bug: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1710571?

[rmkarasev]

I am not sure this issue has impact on ADO.NET DataSet Designer, but just in case, here is the link to a similar issue with the byte[] type in ADO.NET DataSet -> #14388

[Olina-Zhang]

Our internal bug: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1710571 I mentioned above is same as the GH issue: #14388