如何在C＃中使用if / then语句设置变量类型？

Question

I have a function which performs an SQL query. Depending on a registry value, it will either hit SQL Server or SQL Server Compact Edition. If it is using SQL Server CE, the line setting the recordSet variable should read like this:

SqlCeDataReader recordSet = da.ExecuteSQLCommand(selectCommand);

For SQL Server, it should read like this:

SqlDataReader recordSet = da.ExecuteSQLCommand(selectCommand);

I am trying to put all this in an if/then statement at the beginning at the function but cannot seem to figure out how to set the Type within the if/then. Here is my (partial) code:

public static string SqlQuery(string selectCommand, int regval)
{
    var recordSet = null;
    string selectCommand = "select * from whatever";

    if (regval == 0)
    {
        SqlDataReader recordSet = null;
    }
    else
    {
        SqlCEDataReader recordSet = null;
    }

    recordSet = da.ExecuteSQLCommand(selectCommand);
}

The problem is that unless I declare the recordSet variable before the if/else, I cannot use it after. However, if I declare it before the if/else, I cannot change the type.

Answer 1

You can't do that. Variables' types are really set in stone at compile time unless you use dynamic .

You could use the common interface IDataReader . Other standard ADO.NET clients such as for MySQL and SQLite implement this too.

Edit

Here is a sample

public static string SqlQuery(string selectCommand, int regval)
{
    IDataReader recordSet = null;
    string selectCommand = "select * from whatever";
    recordSet = da.ExecuteSQLCommand(selectCommand);
}