You need to provide thread-safe access through accessor functions to an internal member variable.

public static class SaferMemberAccess
{
    private static int _numericField = 1;
    private static object _syncObj = new object( );

    public static void IncrementNumericField( )
    {
        lock (_syncObj)
        {
            ++_numericField;
        }
    }

    public static void ModifyNumericField(int newValue)
    {
        lock (_syncObj)
        {
            _numericField = newValue;
        }
    }

    public static int ReadNumericField( )
    {
        lock (_syncObj)
        {
            return (_numericField);
        }
    }
}