[CSharp] add optional flag "optionalEmitDefaultValues" and update template

Created by: sdoeringNew

PR checklist

• Read the contribution guidelines. But I'm unsure if I named the vendorExtension flag correctly. I do not put the csharp prefix as the other extensions in CSharp did not have this prefix, too.
• Ran the shell script under ./bin/ to update Petstore sample for CSharp... But I haven't committed them because there were many other changes from previous template changes - so nobody seem to commit them. At least I saw that the flag worked as expected.
• Filed the PR against the correct branch: master
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.
• Waited for the checks to have passed.

Description of the PR

This commit addresses issue #3274 (closed) and adds the proposed optionalEmitDefaultValues flag.

Invite cool reviewers

@mandrean (2017/08), @jimschubert (2017/09)

bin/openapi3/csharp-property-changed-petstore.sh

@@ -27,6 +27,6 @@ fi
 
 # if you've executed sbt assembly previously it will use that instead.
 export JAVA_OPTS="${JAVA_OPTS} -Xmx1024M -DloggerPath=conf/log4j.properties"
-ags="generate $@ -i modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml -g csharp -o samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged --additional-properties generatePropertyChanged=true --additional-properties packageGuid={5CD900DE-8266-412F-A758-28E1F9C623D5}"
+ags="generate $@ -i modules/openapi-generator/src/test/resources/3_0/petstore-with-fake-endpoints-models-for-testing.yaml -g csharp -o samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged --additional-properties generatePropertyChanged=true --additional-properties optionalEmitDefaultValues=true --additional-properties packageGuid={5CD900DE-8266-412F-A758-28E1F9C623D5}"
 
 java $JAVA_OPTS -jar $executable $ags

bin/windows/csharp-property-changed-petstore.bat

@@ -5,6 +5,6 @@ If Not Exist %executable% (
 )
 
 REM set JAVA_OPTS=%JAVA_OPTS% -Xmx1024M
-set ags=generate -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g csharp -o samples\client\petstore\csharp\SwaggerClientWithPropertyChanged --additional-properties=generatePropertyChanged=true,packageGuid={5CD900DE-8266-412F-A758-28E1F9C623D5}
+set ags=generate -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g csharp -o samples\client\petstore\csharp\SwaggerClientWithPropertyChanged --additional-properties=generatePropertyChanged=true,optionalEmitDefaultValues=true,packageGuid={5CD900DE-8266-412F-A758-28E1F9C623D5}
 
 java %JAVA_OPTS% -jar %executable% %ags%

bin/csharp-property-changed-petstore.sh

@@ -27,6 +27,6 @@ fi
 
 # if you've executed sbt assembly previously it will use that instead.
 export JAVA_OPTS="${JAVA_OPTS} -Xmx1024M -DloggerPath=conf/log4j.properties"
-ags="generate -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g csharp -o samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged --additional-properties generatePropertyChanged=true --additional-properties packageGuid={5CD900DE-8266-412F-A758-28E1F9C623D5} $@"
+ags="generate -i modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -g csharp -o samples/client/petstore/csharp/OpenAPIClientWithPropertyChanged --additional-properties generatePropertyChanged=true --additional-properties optionalEmitDefaultValues=true --additional-properties packageGuid={5CD900DE-8266-412F-A758-28E1F9C623D5} $@"
 
 java $JAVA_OPTS -jar $executable $ags

docs/generators/csharp.md

@@ -21,6 +21,7 @@ sidebar_label: csharp
 |returnICollection|Return ICollection<T> instead of the concrete type.| |false|
 |optionalMethodArgument|C# Optional method argument, e.g. void square(int x=10) (.net 4.0+ only).| |true|
 |optionalAssemblyInfo|Generate AssemblyInfo.cs.| |true|
+|optionalEmitDefaultValues|Set DataMember's EmitDefaultValue.| |false|
 |optionalProjectFile|Generate {PackageName}.csproj.| |true|
 |generatePropertyChanged|Specifies a AssemblyDescription for the .NET Framework global assembly attributes stored in the AssemblyInfo file.| |false|
 |nonPublicApi|Generates code with reduced access modifiers; allows embedding elsewhere without exposing non-public API calls to consumers.| |false|

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractCSharpCodegen.java

@@ -39,6 +39,7 @@ import static org.openapitools.codegen.utils.StringUtils.camelize;
 public abstract class AbstractCSharpCodegen extends DefaultCodegen implements CodegenConfig {
 
     protected boolean optionalAssemblyInfoFlag = true;
+    protected boolean optionalEmitDefaultValuesFlag = false;
     protected boolean optionalProjectFileFlag = true;
     protected boolean optionalMethodArgumentFlag = true;
     protected boolean useDateTimeOffsetFlag = false;