Add support for CSharpHelper for List literals

Fixes dotnet#19274

Also relates to npgsql/efcore.pg#2402

src/EFCore.Design/Design/Internal/CSharpHelper.cs

@@ -737,6 +737,71 @@ public virtual string Literal(object?[,] values)
         return builder.ToString();
     }
 
+    /// <summary>
+    ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
+    ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
+    ///     any release. You should only use it directly in your code with extreme caution and knowing that
+    ///     doing so can result in application failures when updating to a new Entity Framework Core release.
+    /// </summary>
+    public virtual string Literal<T>(IList<T> values, bool vertical = false)
+        => List(typeof(T), values, vertical);
+
+    private string List(Type type, IEnumerable values, bool vertical = false)
+    {
+        var builder = new IndentedStringBuilder();
+
+        builder.Append("new List<")
+            .Append(Reference(type))
+            .Append("> {");
+
+        var first = true;
+        foreach (var value in values)
+        {
+            if (first)
+            {
+                if (vertical)
+                {
+                    builder.AppendLine();
+                    builder.IncrementIndent();
+                }
+                else
+                {
+                    builder.Append(" ");
+                }
+                first = false;
+            }
+            else
+            {
+                builder.Append(",");
+
+                if (vertical)
+                {
+                    builder.AppendLine();
+                }
+                else
+                {
+                    builder.Append(" ");
+                }
+            }
+
+            builder.Append(UnknownLiteral(value));
+        }
+
+        if (vertical)
+        {
+            builder.AppendLine();
+            builder.DecrementIndent();
+        }
+        else
+        {
+            builder.Append(" ");
+        }
+
+        builder.Append("}");
+
+        return builder.ToString();
+    }
+
     /// <summary>
     ///     This is an internal API that supports the Entity Framework Core infrastructure and not subject to
     ///     the same compatibility standards as public APIs. It may be changed or removed without notice in
@@ -844,6 +909,11 @@ public virtual string UnknownLiteral(object? value)
             return Array(literalType.GetElementType()!, array);
         }
 
+        if (value is IList list && value.GetType().IsGenericType && value.GetType().GetGenericTypeDefinition() == typeof(List<>))
+        {
+            return List(value.GetType().GetGenericArguments()[0], list);
+        }
+
         var mapping = _typeMappingSource.FindMapping(literalType);
         if (mapping != null)
         {
@@ -878,6 +948,35 @@ private bool HandleExpression(Expression expression, StringBuilder builder, bool
 
                 HandleList(((NewArrayExpression)expression).Expressions, builder, simple: true);
 
+                builder
+                    .Append(" }");
+
+                return true;
+            case ExpressionType.ListInit:
+                var listExpr = (ListInitExpression)expression;
+                if (listExpr.Initializers.Any(i => i.Arguments.Count != 1))
+                {
+                    // If there is an initializer with more than one argument we can't make a literal cleanly
+                    return false;
+                }
+
+                builder
+                    .Append("new ")
+                    .Append(Reference(expression.Type));
+
+                if (listExpr.NewExpression.Arguments.Count > 0 && !HandleArguments(listExpr.NewExpression.Arguments, builder))
+                {
+                    return false;
+                }
+
+                builder
+                    .Append(" { ");
+
+                if (!HandleList(listExpr.Initializers.Select(i => i.Arguments.First()), builder, simple: true))
+                {
+                    return false;
+                }
+
                 builder
                     .Append(" }");
 

test/EFCore.Design.Tests/Design/Internal/CSharpHelperTest.cs

@@ -121,6 +121,24 @@ public void Literal_works_when_many_ByteArray()
             new byte[] { 1, 2 },
             "new byte[] { 1, 2 }");
 
+    [ConditionalFact]
+    public void Literal_works_when_empty_list()
+        => Literal_works(
+            new List<string>(),
+            @"new List<string> { }");
+
+    [ConditionalFact]
+    public void Literal_works_when_list_without_ctor_arguments()
+        => Literal_works(
+            new List<string> { "one", "two" },
+            @"new List<string> { ""one"", ""two"" }");
+
+    [ConditionalFact]
+    public void Literal_works_when_list_with_ctor_arguments()
+        => Literal_works(
+            new List<string>(new [] { "one" }) { "two", "three" },
+            @"new List<string> { ""one"", ""two"", ""three"" }");
+
     [ConditionalFact]
     public void Literal_works_when_multiline_string()
         => Literal_works(
@@ -607,6 +625,33 @@ public void Literal_with_add()
             new CSharpHelper(typeMapping).UnknownLiteral(new SimpleTestType()));
     }
 
+    [ConditionalFact]
+    public void Literal_with_list_of_string_init_no_arguments()
+    {
+        var typeMapping = CreateTypeMappingSource<SimpleTestType>(
+            v => Expression.ListInit(
+                Expression.New(typeof(List<>).MakeGenericType(typeof(string))),
+                Expression.Constant("one"), Expression.Constant("two")));
+
+        Assert.Equal(
+            @"new List<string> { ""one"", ""two"" }",
+            new CSharpHelper(typeMapping).UnknownLiteral(new SimpleTestType()));
+    }
+
+    [ConditionalFact]
+    public void Literal_with_list_of_string_init_with_arguments()
+    {
+        var constructor = typeof(List<>).MakeGenericType((typeof(string))).GetConstructor(new[] { typeof(IEnumerable<string>) })!;
+        var typeMapping = CreateTypeMappingSource<SimpleTestType>(
+            v => Expression.ListInit(
+                Expression.New(constructor, Expression.NewArrayInit(typeof(string), Expression.Constant("one"))),
+                Expression.Constant("one"), Expression.Constant("two")));
+
+        Assert.Equal(
+            @"new List<string>(new string[] { ""one"" }) { ""one"", ""two"" }",
+            new CSharpHelper(typeMapping).UnknownLiteral(new SimpleTestType()));
+    }
+
     [ConditionalFact]
     public void Literal_with_unsupported_node_throws()
     {