Import Missing Namespaces
When you use types whose namespaces have not been imported in the file, JetBrains Rider helps you locate these types and add the missing namespace import directives. If there are several missing namespaces for an unresolved type, for example, after you paste a block of code in the file, JetBrains Rider would help you import all these namespaces in a single action.
JetBrains Rider looks for non-imported namespaces in the following assemblies: assemblies referenced in the current project, assemblies referenced in other projects in the solution, assemblies that are referenced transitively. If the type is found in a module that is not referenced in the current project, JetBrains Rider also suggests adding a reference to this module.
The missing namespaces are imported according to your namespace import preferences (for example, import directives are inserted or type names qualifiers are added).
The import directives are added in a fixed order: all System.* namespaces go first, sorted alphabetically by the second word after the dot; all other namespaces go next, in alphabetical order.
When you edit a code file, types with missing namespaces are detected with the design-time code inspection and highlighted as errors. By default, the action that imports missing namespaces differs from the normal quick-fix: it is displayed as a popup that hovers either over the first unresolved symbol regardless of your caret position:
or after a pasted code bock:
Thus, you can apply this fix for all unresolved types with the habitual Alt+Enter shortcut without changing your current caret position, so your editing session is not interrupted. Note that the Import types for pasted code action only appears if all non-imported types in the pasted code block can be unambiguously found in the referenced modules of the current project.
In case several namespace contain the unresolved symbol, JetBrains Rider lets you choose the namespace to import:
If for some reason you chose not to import a required namespace when the popup was displayed, or if you disabled Show the "Import namespace" action using popup on the page of JetBrains Rider settings Ctrl+Alt+S, you can import a type at any time by putting the caret at the non-imported type, pressing Alt+Enter and choosing the corresponding quick-fix. 
To quickly choose whether to show import suggestions via the popup, or an entry in the Alt+Enter menu, use the context menu of the file status indicator:
You may want some namespaces not to be suggested, for example, if you have something similar to a system type in your solution, say MyFramework.MyCollections.List, but you are not actually using it. To exclude such symbols from the suggestions, add them to the Exclude from import and completion list on the page of JetBrains Rider settings Ctrl+Alt+S. The format of the entries is Fully.Qualified.Name, Fully.Qualified.Name.Prefix*, or *Fully.Qualified.Name.Suffix. Generic types are specified as List`1.