fix(C#): added dependency usings based on types present in renderContext

packages/quicktype-core/src/language/CSharp/CSharpRenderer.ts

@@ -24,6 +24,7 @@ import type {
 } from "../../Type";
 import {
     directlyReachableSingleNamedType,
+    matchCompoundType,
     matchType,
     nullableFromUnion,
     removeNullFromUnion,
@@ -526,4 +527,25 @@ export class CSharpRenderer extends ConvenienceRenderer {
 
         this.emitDefaultFollowingComments();
     }
+
+    protected emitDependencyUsings(): void {
+        let genericEmited: boolean = false;
+        let ensureGenericOnce = () => {
+            if (!genericEmited) {
+                this.emitUsing("System.Collections.Generic");
+                genericEmited = true;
+            }
+        }
+        this.typeGraph.allTypesUnordered().forEach(_ => {
+            matchCompoundType(
+                _,
+                _arrayType => this._csOptions.useList ? ensureGenericOnce() : undefined,
+                _classType => { },
+                _mapType => ensureGenericOnce(),
+                _objectType => { },
+                _unionType => { }
+            )
+        });
+    }
+
 }

packages/quicktype-core/src/language/CSharp/NewtonSoftCSharpRenderer.ts

@@ -150,8 +150,10 @@ export class NewtonsoftCSharpRenderer extends CSharpRenderer {
     }
 
     protected emitUsings(): void {
-        // FIXME: We need System.Collections.Generic whenever we have maps or use List.
-        if (!this._needAttributes && !this._needHelpers) return;
+        if (!this._needAttributes && !this._needHelpers) {
+            this.emitDependencyUsings();
+            return;
+        }
 
         super.emitUsings();
         this.ensureBlankLine();

packages/quicktype-core/src/language/CSharp/SystemTextJsonCSharpRenderer.ts

@@ -152,8 +152,10 @@ export class SystemTextJsonCSharpRenderer extends CSharpRenderer {
     }
 
     protected emitUsings(): void {
-        // FIXME: We need System.Collections.Generic whenever we have maps or use List.
-        if (!this._needAttributes && !this._needHelpers) return;
+        if (!this._needAttributes && !this._needHelpers) {
+            this.emitDependencyUsings();
+            return;
+        }
 
         super.emitUsings();
         this.ensureBlankLine();