A easy question about LINK2019 in ProJucer

I want to learning JUCE.

From the example ,I download tutorials example projects which used in ProJucer…

But when I open it in VS2017 and building it …

I opened project is “Control Audio Level”,

I dont know why beeause it successed running yesterday…

The IDE tell me error LINK2019:unresolved external symbol:
1>MainComponent.obj : error LNK2019: 无法解析的外部符号 “public: __cdecl juce::AudioAppComponent::AudioAppComponent(void)” (??0AudioAppComponent@juce@@QEAA@XZ),该符号在函数 “public: __cdecl MainContentComponent::MainContentComponent(void)” (??0MainContentComponent@@QEAA@XZ) 中被引用

That’s a linker error. It looks like perhaps you haven’t provided a definition for your MainContentComponent constructor? Can you compare your code to the unedited tutorial project and see where it’s different?