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In the net_v40_long and previous versions, by default, the UI culture of all threads is set to the Windows system culture. For applications whose current UI culture differs from the default system culture, this behavior is often undesirable. In the net_v45, the CultureInfo.DefaultThreadCurrentUICulture property lets you define the default UI culture of all threads in an application domain.
If you have not explicitly set the UI culture of any existing threads executing in an application domain, setting the CultureInfo.DefaultThreadCurrentUICulture property also changes the culture of these threads. However, if these threads execute in another application domain, their culture is defined by the CultureInfo.DefaultThreadCurrentUICulture property in that application domain or, if no default value is defined, by the default system culture. Because of this, we recommend that you always explicitly set the culture of your main application thread and do not rely on the CultureInfo.DefaultThreadCurrentUICulture property to define the culture of the main application thread.
Unless it is set explicitly, the value of the CultureInfo.DefaultThreadCurrentUICulture property is null, and the current culture of all threads in an application domain that have not been assigned an explicit culture is defined by the default Windows system culture.
For more information about cultures, threads, and application domains, see the "Culture and Threads" and "Culture and Application Domains" sections in the System.Globalization.CultureInfo class topic.