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os.environ contains all environment variables uppercase on Windows which cause the key in the self.MacroDictionary is uppercase, but the real variable name maybe mixed case, eg:WINSDK81x86, then we can't find the variable in the self.MacroDictionary. Cc: Liming Gao <liming.gao@intel.com> Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com> Reviewed-by: Liming Gao <liming.gao@intel.com>