C＃泛型实例化

Question

All, I have a method that returns a List. This method is used to return the parameters of SQL StoredProcedures, Views and Functions depending on name. What I want to do is create a list of objects and return this list to the caller. The method is below

private List<T> GetInputParameters<T>(string spFunViewName)
{
    string strSql = String.Format(
        "SELECT PARAMETER_NAME, DATA_TYPE FROM INFORMATION_SCHEMA.PARAMETERS " +
        "WHERE SPECIFIC_NAME = '{0}' AND PARAMETER_MODE = 'IN';",
        spFunViewName);
    List<string[]> paramInfoList = new List<string[]>();
    DataTable paramDt = Utilities.DTFromDB(conn, "InputParmaters", strSql);
    if (paramDt != null)
    {
        Converter<DataRow, string[]> rowConverter =
            new Converter<DataRow, string[]>(Utilities.RowColConvert);
        paramInfoList = Utilities.ConvertRowsToList<string[]>(paramDt, rowConverter);
    }
    else
        return null;

    // Build the input parameter list.
    List<T> paramList = new List<T>();
    foreach (string[] paramInfo in paramInfoList)
    {
        T t = new T(paramInfo[NAME], paramInfo[TYPE], Convert.ToInt32(paramInfo[CHARMAXLEN]));
        columnList.Add(column);
    }
    return columnList;   
}

I clearly can't instantiate T via new and pass to the constructor, but it should be clear what I am attempting to do. Is there a way to do what I want with out three additional methods?

Note. The main issue is that the number of parameters I am passing to T can either be two OR three.

Thanks for your time.

Edit: The struct s I use are as follows

public struct Database
{
    public string name { get; set; }
    public string filename { get; set; }
    public List<Table> tables { get; set; }
    public List<StoredProcedure> sps { get; set; }
    public List<Function> funcs { get; set; }
    public List<View> views { get; set; }
    public Database(string name, string filename)
    {
        this.name = name;
        this.filename = filename;
    }
}

protected internal struct StoredProcedure
{
    public string name { get; set; }
    public List<string[]> parameters { get; set; }
    public StoredProcedure(string name, List<string[]> parameters)
    {
        this.name = name;
        this.parameters = parameters;
    }
}

protected internal struct Function
{
    public string name { get; set; }
    public string output { get; set; }
    public List<string[]> parameters { get; set; }
    public Function(string name, string output, List<string[]> parameters)
    {
        this.name = name;
        this.output = output;
        this.parameters = parameters;
    }
}

protected internal struct View
{
    public string name {get; set;} 
    public List<string[]> parameters { get; set; }
    public View(string name, List<string[]> parameters)
    {
        this.name = name;
        this.parameters = parameters;
    }
}

Answer 1

Use the Activator class to create T and pass the parameters.

Type type = typeof(T);
var result = (T)Activator.CreateInstance(type, new object[] { yourParameters });

Used in your code snippet:

T t = Activator.CreateInstance(type, colInfo[NAME], colInfo[TYPE], Convert.ToInt32(colInfo[CHARMAXLEN]));

Answer 2

I'm neither endorsing nor detracting this technique, but you can use:

(T) Activator.CreateInstance( typeof(T), colInfo[TYPE], Convert.ToInt32(colInfo[CHARMAXLEN]) );

I think I'd rather have separate factory methods.

Answer 3

You can use Activator.CreateInstance() as other mentioned or pass a delegate Func<string, string, int, T> avoiding the reflection overhead .

 List<T> GetInputParameters<T>(string spFunViewName, Func<string, string, int, T> itemCreator)
 {

    ....
    List<T> paramList = new List<T>();     
    foreach (string[] paramInfo in paramInfoList)     
    {         
       T t = itemCreator(paramInfo[NAME], paramInfo[TYPE], 
            Convert.ToInt32(paramInfo[CHARMAXLEN]));         
      paramList.Add(t);     
    }     

    return columnList;    
 }

Answer 4

I clearly can't instansiate T via new and pass to the constructor

As written, no; however, you can if you constrain your type parameter to only accept types with constructors:

private List<T> GetInputParameters<T>(string spFunViewName) where T : new()
{
    // your code here
}

In the example above you would be able to say:

T myItem = new T();

In your specific case, it looks like you expect each of the generic types to share something in common. Consider also constraining the type with an interface:

private List<T> GetInputParameters<T>(string spFunViewName) where T : new(), ISomeInterface
{
    // your code here
}

That would allow you to, after you instantiate you object, apply values to any properties on the interface:

T myItem = new T();

myItem.SomeProperty = somevalue;
myItem.AnotherProperty = anothervalue;

For more info, check out Constraints on Type Parameters (C# Programming Guide) on MSDN for more info on generic type constraints.

Answer 5

You could just use List< DbParameter >

That is a bit more obvious.

Answer 6

You could try this:

var constructor = typeof(T).GetConstructor(typeof(string), typeof(string), typeof(int));
constructor.Invoke(colInfo[NAME], colInfo[TYPE], Convert.ToInt32(colInfo[CHARMAXLEN]));

Answer 7

You can create generic database parameters (and connections and commands, etc) with ADO.NET DbProviderFactories .

The System.Data.Common namespace provides classes for creating DbProviderFactory instances to work with specific data sources. When you create a DbProviderFactory instance and pass it information about the data provider, the DbProviderFactory can determine the correct, strongly typed connection object to return based on the information it has been provided.

In your code, you could create a DbProviderFactory and then call CreateParameter() .

string providerName = ConfigurationManager.ConnectionStrings["YourConnectionString"].ProviderName;
DbProviderFactory factory = DbProviderFactories.GetFactory(providerName);
DbParameter parameter = factory.CreateParameter();