出现这样的问题是因为MCV4 默认Newtonsoft.Json 4.5 而SingalR是需要版本6.0
解决办法是在webconfig runtime 中加入下面代码
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-8.0.0.0" newVersion="6.0.0.0" />
</dependentAssembly>
</assemblyBinding>