How to read a user environment variable in C#?

Use the other overload of the Environment.GetEnvironmentVariable Method that lets you specify the EnvironmentVariableTarget.

Environment.GetEnvironmentVariable(variable, target);

target can be:
EnvironmentVariableTarget.Process,
EnvironmentVariableTarget.User,
EnvironmentVariableTarget.Machine.

It's the same method, just set the second parameter to be User as:

System.Environment.GetEnvironmentVariable("varName", EnvironmentVariableTarget.User);