Do not scaffold a default constructor when OnConfiguring is suppressed

Version 5.0.0 of the dotnet-ef tool introduced the `--no-onconfiguring` option which is great if you plan to configure your DbContext outside of the context itself (which you probably should).

When using this option, the DbContext class is still generated with a default constructor. This default constructor becomes problematic without the `OnConfiguring` method since the context is unusable when created through the default constructor. As soon as you try to use a DbContext created by the default constructor, you get a `System.InvalidOperationException`:
> No database provider has been configured for this DbContext. A provider can be configured by overriding the 'DbContext.OnConfiguring' method or by using 'AddDbContext' on the application service provider. If 'AddDbContext' is used, then also ensure that your DbContext type accepts a DbContextOptions<TContext> object in its constructor and passes it to the base constructor for DbContext.

Fixes dotnet#23515

src/EFCore.Design/Scaffolding/Internal/CSharpDbContextGenerator.cs

@@ -158,7 +158,7 @@ protected virtual void GenerateClass(
 
  using (_builder.Indent())
  {
- GenerateConstructors(contextName);
+ GenerateConstructors(contextName, generateDefaultConstructor: !suppressOnConfiguring);
  GenerateDbSets(model);
  GenerateEntityTypeErrors(model);
  if (!suppressOnConfiguring)
@@ -179,12 +179,15 @@ protected virtual void GenerateClass(
  _builder.AppendLine("}");
  }
 
- private void GenerateConstructors(string contextName)
+ private void GenerateConstructors(string contextName, bool generateDefaultConstructor)
  {
- _builder.AppendLine($"public {contextName}()")
- .AppendLine("{")
- .AppendLine("}")
- .AppendLine();
+ if (generateDefaultConstructor)
+ {
+ _builder.AppendLine($"public {contextName}()")
+ .AppendLine("{")
+ .AppendLine("}")
+ .AppendLine();
+ }
 
  _builder.AppendLine($"public {contextName}(DbContextOptions<{contextName}> options)")
  .IncrementIndent()

test/EFCore.Design.Tests/Scaffolding/Internal/CSharpDbContextGeneratorTest.cs

@@ -146,10 +146,6 @@ namespace TestNamespace
 {
  public partial class TestDbContext : DbContext
  {
- public TestDbContext()
- {
- }
-
  public TestDbContext(DbContextOptions<TestDbContext> options)
  : base(options)
  {
@@ -1047,10 +1043,6 @@ namespace TestNamespace
 {
  public partial class TestDbContext : DbContext
  {
- public TestDbContext()
- {
- }
-
  public TestDbContext(DbContextOptions<TestDbContext> options)
  : base(options)
  {

test/EFCore.Design.Tests/Scaffolding/Internal/ModelCodeGeneratorTestBase.cs

@@ -63,14 +63,15 @@ protected void Test(
  if (!skipBuild)
  {
  var assembly = build.BuildInMemory();
- var contextNamespace = options.ContextNamespace ?? options.ModelNamespace;
- var context = (DbContext)assembly.CreateInstance(
- !string.IsNullOrEmpty(contextNamespace)
- ? contextNamespace + "." + options.ContextName
- : options.ContextName);
 
  if (assertModel != null)
  {
+ var contextNamespace = options.ContextNamespace ?? options.ModelNamespace;
+ var context = (DbContext)assembly.CreateInstance(
+ !string.IsNullOrEmpty(contextNamespace)
+ ? contextNamespace + "." + options.ContextName
+ : options.ContextName);
+
  var compiledModel = context.GetService<IDesignTimeModel>().Model;
  assertModel(compiledModel);
  }