C# conditional using block statement

One option, which is somewhat nasty but would work, based on the fact that the C# compiler calls Dispose only if the resource is non-null:

protected void ValidateExportDirectoryExists()
{
    using (useNetworkAccess 
               ? new Core.NetworkAccess(username, password, domain)
               : null)
    {
        CheckExportDirectoryExists();
    }
}

Another alternative would be to write a static method which returned either null or a NetworkAccess:

private Core.NetworkAccess CreateNetworkAccessIfNecessary()
{
    return useNetworkAccess
        ? new Core.NetworkAccess(username, password, domain)) : null;
}

Then:

protected void ValidateExportDirectoryExists()
{
    using (CreateNetworkAccessIfNecessary())
    {
        CheckExportDirectoryExists();
    }
}

Again, I’m still not sure I don’t prefer the original… it really depends on how often you need this pattern.