Some computers may have security setup to not trust some files download from the web such as a form resource file in a VB.NET or C# project. If there are many files one must right click on each file in Windows Explorer and un-check mark of the web. If not done Visual Studio will complain and not compile the project.

The following code accepts a path to were these files exists and unmark all files in the root and underlying folders.

C# 9 .NET Core

Code:
/// <summary>
/// Removed 'mark of the web' from all files in a folder recursively 
/// </summary>
/// <param name="folderName">folder to perform the operation on</param>
public static void UnblockFiles(string folderName)
{
    if (!Directory.Exists(folderName))
    {
        return;
    }

    var start = new ProcessStartInfo
    {
        FileName = "powershell.exe",
        Arguments = $"Get-ChildItem -Path '{folderName}' -Recurse | Unblock-File",
        CreateNoWindow = true
    };

    using var process = Process.Start(start);

}
VB.NET

Code:
''' <summary>
''' Removed 'mark of the web' from all files in a folder recursively 
''' </summary>
''' <param name="folderName">folder to perform the operation on</param>
Public Sub UnblockFiles(folderName As String)
    If Not Directory.Exists(folderName) Then
        Return
    End If

    Dim start = New ProcessStartInfo With {
            .FileName = "powershell.exe",
            .Arguments = $"Get-ChildItem -Path '{folderName}' -Recurse | Unblock-File",
            .CreateNoWindow = True
            }

    Using process As Process = Process.Start(start)

    End Using
End Sub
Or simply open a PowerShell window and run the script in Arguments.