Thanks Ken, I got it to work. The problem was with this line:
Dim pEditor As ESRI.ArcGIS.Editor.IEditor
pEditor = m_application.FindExtensionByName("esriEditor.Editor")
I ended up using:
Dim pEditor As ESRI.ArcGIS.Editor.IEditor
pEditor = My.ArcMap.Application.FindExtensionByName("esriEditor.Editor")
The whole code:
Imports ESRI.ArcGIS.esriSystem
Imports ESRI.ArcGIS.Geometry
Imports ESRI.ArcGIS.Geodatabase
Imports ESRI.ArcGIS.Carto
Imports ESRI.ArcGIS.ArcMapUI
Imports ESRI.ArcGIS.Framework
Imports System.Windows.Forms
Imports ESRI.ArcGIS.Editor
Public Class EaseRowEditorForm
Inherits System.Windows.Forms.Form
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim pEnumFeature As ESRI.ArcGIS.Geodatabase.IEnumFeature
Dim pFeature As ESRI.ArcGIS.Geodatabase.IFeature
Dim pEditor As ESRI.ArcGIS.Editor.IEditor
pEditor = My.ArcMap.Application.FindExtensionByName("esriEditor.Editor")
pEnumFeature = pEditor.EditSelection
pEnumFeature.Reset()
pFeature = pEnumFeature.Next
Do Until pFeature Is Nothing
pFeature.Value(pFeature.Fields.FindField("Source")) = "Coordinates"
pFeature.Store()
pFeature = pEnumFeature.Next
Loop
End Sub
End Class