Why does a case statement NOT allow me to set variables that will be used inside a method

Question

Im trying to use a case statement to assign a value to a variable named "query". Depending on the value of a comboBox, the value of query will change. I assigned the "query" variable inside my method and want to use it within the method only. I get an error message that the "query" variable is unassigned even though it is being assigned at the top of the method. I have a work around but I dont know why this happens? any insight would be helpfull.

heres the code.

public void ExportKml()
    {
        string query;

            switch (txtTable.SelectedIndex)
            {
                case 0:
                    query = "Select * from dbo.HyacinthWaterBodyZones";
                    break;
                case 1:
                    query="Select * from lchcd.privateWatersFinal where waterbodypolygon is not null";
                    break;
                case 2:
                    query = "Select * from lchcd.publicWatersFinal where waterbodypolygon is not null";
                    break;

            }

            cs.Open();
            SqlCommand cmd = new SqlCommand(query, cs);   <<--Error Message 
            SqlDataReader polygon = cmd.ExecuteReader();
}

the "query" variable inside the line: SqlCommand cmd = new SqlComman(query,cs) gives an error stating it is unassigned local variable.

Answer 1

You need to asign an initial value to query. Either "" or string.Empty. You should include a default case in your switch also. See below:

public void ExportKml()
{
    string query = string.Empty;

    switch (txtTable.SelectedIndex)
    {
        case 0:
            query = "Select * from dbo.HyacinthWaterBodyZones";
            break;
        case 1:
            query = "Select * from lchcd.privateWatersFinal where waterbodypolygon is not null";
            break;
        case 2:
            query = "Select * from lchcd.publicWatersFinal where waterbodypolygon is not null";
            break;
        default:
            query = "";
            break;
    }

    // Add a check for empty string before trying the query.
    if(!string.IsNullOrWhiteSpace(query))
    {
        cs.Open();
        SqlCommand cmd = new SqlCommand(query, cs);
        SqlDataReader polygon = cmd.ExecuteReader();
    }
}

Answer 2

Because the compiler has no way to know if your SelectedIndex will be always 0,1,or 2, so it gives you a strong suggestion to initialize the variable defined before the switch but used after it

string query = string.Empty;
switch(.....)
{
    ....
}
if(query.Length > 0)
{
    cs.Open();
    SqlCommand cmd = new SqlCommand(query, cs);   <<--Error Message 
    SqlDataReader polygon = cmd.ExecuteReader();    
}

Adding a default case to the switch statement works also, but, personally, I prefer to initialize the query variable before entering the switch statement. It is more clear to me and less prone to forgetting some other important initializations

Answer 3

If the selected index is not 0-2, you will not hit any of the case statements. You need to include a default: case if you are going to assign a variable in the switch.

switch (txtTable.SelectedIndex)
{
    case 0:
        query = "Select * from dbo.HyacinthWaterBodyZones";
        break;
    case 1:
        query="Select * from lchcd.privateWatersFinal where waterbodypolygon is not null";
        break;
    case 2:
        query = "Select * from lchcd.publicWatersFinal where waterbodypolygon is not null";
        break;
    default:
        //assign query = something here
        break;
}