How to get float value with SqlDataReader?

问题

In my database, I have NextStatDistanceTime value as a float. When "float time = reader.GetFloat(0);" line excecuted, it gives an error of

system invalid cast exception

How can I get float value from sql command in this code?

Here is my code:

using (SqlConnection conn = new SqlConnection(@"<myconnectionstring>"))
{
    float totaltime = 0;
    for (int i = startStationIndex; i < endStationIndex; i++)
    {
        SqlCommand command = new SqlCommand("SELECT NextStatDistanceTime FROM [MetroDatabase].[dbo].[MetroStation] WHERE StationIndex = " + i + "", conn);
        try
        {
            conn.Open();
            command.ExecuteNonQuery();
            using (SqlDataReader reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    float time = reader.GetFloat(0);
                    totaltime = totaltime + time;
                    conn.Close();
                }
            }                        
        }
        catch (Exception ex)
        {
            result = ex.Message;
            Console.WriteLine(ex.Message);
        }
    }

}

回答1:

It's time for a little table, I think.

T-SQL type name | .NET equivalent | C# type name | DataReader method
----------------+-----------------+--------------+------------------------
FLOAT           | System.Double   | double       | IDataReader.GetDouble()
REAL            | System.Single   | float        | IDataReader.GetFloat()

Note that GetFloat has the wrong name -- it should be GetSingle, because float is a C#-specific name. It makes no sense in VB.NET, for example.

So, if your database column is of type FLOAT, read it using GetDouble, not GetFloat. The data reader methods do not perform conversions; there is a generic GetValue method to get the value as an object that you can then convert further.

Incidentally, this is not the only subtlety -- the .NET floating-point types support denormalized values, whereas the T-SQL types do not, so it is possible to have floating-point numbers in your .NET code that can't be successfully stored in the database, even if the types match.

回答2:

As you can read here a sql-server float maps to a .NET double, so you need to use GetDouble:

double totaltime = 0;  // necessary, double is wider than float
// ...

while (reader.Read())
{
    double time = reader.GetDouble(0);
    totaltime = totaltime + time;
    // conn.Close(); no, not in this loop, should be closed in the finally or via using-statement
}

回答3:

My guess is that Database is returning double value, try getting it as Double and convert it float (if required).

float time= (float) reader.GetDouble(0);

回答4:

you can try:

float time = float.Parse(reader[0].ToString());

also note (though not related with your Q) that you don't need to run

command.ExecuteNonQuery();

回答5:

Try this

convert.ToSingle(reader["NextStatDistanceTime"])

or do

double value = (double)reader["NextStatDistanceTime"]

Float of sql is equivalent to double of c#, you can see the similar mapping here

回答6:

 while (reader.Read())
 {
     object initialTime = reader["NextStatDistanceTime"];
     float time;
     float.TryParse(initialTime.ToString(), out time);

     totaltime = totaltime + time;
     conn.Close();
 }

Try this, this will get the time from the Database then convert it to a float, you can just put the reader["NextStatDistanceTime] in the tryparse if you want but to make it clearer i have done it like this.

Any issues let me know

回答7:

Probably is precision mismatch between database type and c# type. Try cast like (float)reader.GetDouble(0);

来源：https://stackoverflow.com/questions/37344167/how-to-get-float-value-with-sqldatareader

标签

sql

casting

reader