If you've been using the class "CrmConnection" in the Microsoft.Xrm.Client assembly, starting with the CRM 2016 (and subsequent) SDK versions (version 8.x), it's no longer available. Instead, you'll need to connect using the CrmServiceClient class which is found in the Microsoft.Xrm.Tooling.Connector assembly. You can view the details here. Note that the matrix on that page points out that you can use the v6.1.2 SDK client with CRM versions 6.x, v7.x, and v8.x.
You can find more information about the CrmServiceClient class here and here. For examples of various connection strings use and connect using the CrmServiceClient class, visit here. This page has links to various examples of creating, retrieving, updating and deleting data using the XRM Tooling model.
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