Using SendStringToExecute() to do what you're seeking to do essentially amounts to a horror show. It doesn't execute the commands you pass it until your code exits and returns control to AutoCAD, so it is not a solution to this (or most, for that matter) problem.
The example code below requires the ContextManager.cs file that is attached. It was designed from the ground up to support batch processing of DWG files without the limitations imposed by code that runs exclusively in the application context (where your code is running). I modified it from the original example code (from this post), to do what your code is attempting to do (draw a circle and save the file).
If you add the ContextManager.cs file to your project, and replace the button4_Click handler in that file with the one shown below, and run it from the palette set created and shown by the example code (shown by using the DCPALETTE command), it should do exactly what the code you posted intended to do.
I haven't tested ContextManager.cs on recent AutoCAD releases, but since AutoCAD and it's managed API have been stagnant for so long, I don't see any reason why it wouldn't work on any release prior to AutoCAD 2024 (which it probably won't work on as-is). If you decide to try it and it doesn't build, let me know what the issue(s) are.
/// <summary>
/// The click event for button4 demonstrates how to use
/// the second overload of the ForEach() extension method
/// to batch-process multiple DWG files selected from a
/// file dialog.
/// </summary>
/// <remarks>
/// Each file in the argument list is sequentially opened
/// and the delegate invoked with the Document representing
/// the open file. When the delegate returns, the document
/// is closed, and processing proceeds to the next file.
///
/// The process performed on each file:
///
/// 1. Draw a circle at 2,2,0 with a radius of 4.0 units
/// 3. Save the file
///
/// The delegate that performs the processing of each file
/// executes in the document context, allowing the use of
/// the Command() method. The delegate must save the file
/// if necessary, as it will not be saved automatically,
/// and any changes made by the delegate will be discarded.
/// </remarks>
void button4_Click(object sender, EventArgs e)
{
OpenFileDialog ofd = new OpenFileDialog();
ofd.AddExtension = true;
ofd.Filter = "Drawing files|*.dwg";
ofd.FilterIndex = 0;
ofd.Multiselect = true;
ofd.DefaultExt = ".dwg";
if(ofd.ShowDialog() == DialogResult.OK && ofd.FileNames.Length > 0)
{
Application.DocumentManager.ForEach(ofd.FileNames,
delegate(Document doc)
{
doc.Editor.Command("._CIRCLE", new Point3d(2.0, 2.0, 0.0), 4.0);
doc.Save();
}
);
}
}
