Скачать или смотреть C# : You must add a reference to assembly mscorlib, version=4.0.0

C# : You must add a reference to assembly mscorlib, version=4.0.0
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C# : You must add a reference to assembly mscorlib, version=4.0.0
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a must : C# add assembly version=4.0.0 reference to You mscorlib,