Unable to inherit 'SqlScriptGenerator' class

从 issue 中显示的 SqlScriptGenerator、SqlScriptGeneratorVisitor、ScriptWriter 和 Sql100ScriptGenerator 签名开始。检查当前如何构造 visitor 和 writer，然后确定外部子类能够编译所需的 public/protected API 更改。当自定义 SqlScriptGenerator 能够继承并 override visitor factory，而不依赖内部类型时，即视为完成。

Is your feature request related to a problem? Please describe.

Consider the following code to inherit SqlScriptGenerator

internal class CustomSqlScriptGenerator : SqlScriptGenerator
{
	public CustomSqlScriptGenerator(SqlScriptGeneratorOptions options) : base(options)
	{
	}

	internal override SqlScriptGeneratorVisitor CreateSqlScriptGeneratorVisitor(SqlScriptGeneratorOptions options, ScriptWriter scriptWriter)
	{
		throw new NotImplementedException();
	}
}

However, this will get compilation error because SqlScriptGeneratorVisitor and ScriptWriter are marked as internal, which is impossible to inherit this class.

Describe the solution you'd like

• Change CreateSqlScriptGeneratorVisitor(...) become protected instead of internal
• Remove ScriptWriter parameter in CreateSqlScriptGeneratorVisitor(...). This can avoid to mark ScriptWriter as public. Instantiation of ScriptWriter will be handled by child class of SqlScriptGenerator. See example below.
• Mark SqlScriptGeneratorVisitor as public accessible. SqlScriptGeneratorVisitor will be used to visit the expression tree when generating TSQL script.

Here is the suggested changes:

protected abstract SqlScriptGeneratorVisitor CreateSqlScriptGeneratorVisitor(SqlScriptGeneratorOptions options);

And update Sql###ScriptGenerator become

Example:

public sealed class Sql100ScriptGenerator : SqlScriptGenerator
{
    public Sql100ScriptGenerator()
        : this(new SqlScriptGeneratorOptions())
    {
    }

    public Sql100ScriptGenerator(SqlScriptGeneratorOptions options)
        : base(options)
    {
    }

    protected override SqlScriptGeneratorVisitor CreateSqlScriptGeneratorVisitor(SqlScriptGeneratorOptions options)
    {
        // Create instance of ScriptWriter. 
        //Currently this is created by private function in SqlScriptGenerator
        ScriptWriter scriptWriter = new ScriptWriter(options);

        ScriptGeneratorSupporter.CheckForNullReference((object) options, nameof (options));
        ScriptGeneratorSupporter.CheckForNullReference((object) scriptWriter, nameof (scriptWriter));
        return (SqlScriptGeneratorVisitor) new Sql100ScriptGeneratorVisitor(options, scriptWriter);
    }
}