create JSON string from SqlDataReader

Question

UPDATE

I figured it out. Check out my answer below.

---

I'm trying to create a JSON string representing a row from a database table to return in an HTTP response. It seems like Json.NET would be a good tool to utilize. However, I'm not sure how to do build the JSON string while I'm reading from the database.

The problem is marked by the obnoxious comments /******** ********/

// connect to DB
theSqlConnection.Open(); // open the connection

SqlDataReader reader = sqlCommand.ExecuteReader();
if (reader.HasRows) {

    while(reader.Read()) {

        StringBuilder sb = new StringBuilder();
        StringWriter sw = new StringWriter(sb);

        using (JsonWriter jsonWriter = new JsonTextWriter(sw)) {

            // read columns from the current row and build this JsonWriter
            jsonWriter.WriteStartObject();
            jsonWriter.WritePropertyName("FirstName");

            // I need to read the value from the database
/******** I can't just say reader[i] to get the ith column. How would I loop here to get all columns? ********/
            jsonWriter.WriteValue(... ? ...);
            jsonWriter.WritePropertyName("LastName");
            jsonWriter.WriteValue(... ? ...);
            jsonWriter.WritePropertyName("Email");
            jsonWriter.WriteValue(... ? ...);

            // etc...
            jsonWriter.WriteEndObject();
        }
    }
}

The problem is that I don't know how to read each column from the row from the SqlReader such that I can call WriteValue and give it the correct information and attach it to the correct column name. So if a row looks like this...

| FirstName | LastName | Email |

... how would I create a JsonWriter for each such row such that it contains all column names of the row and the corresponding values in each column and then use that JsonWriter to build a JSON string that is ready for returning through an HTTP Response?

Let me know if I need to clarify anything.

Solution 1

Got it! Here's the C#...

// ... SQL connection and command set up, only querying 1 row from the table
StringBuilder sb = new StringBuilder();
StringWriter sw = new StringWriter(sb);
JsonWriter jsonWriter = new JsonTextWriter(sw);

try {

    theSqlConnection.Open(); // open the connection

    // read the row from the table
    SqlDataReader reader = sqlCommand.ExecuteReader();
    reader.Read();

    int fieldcount = reader.FieldCount; // count how many columns are in the row
    object[] values = new object[fieldcount]; // storage for column values
    reader.GetValues(values); // extract the values in each column

    jsonWriter.WriteStartObject();
    for (int index = 0; index < fieldcount; index++) { // iterate through all columns

        jsonWriter.WritePropertyName(reader.GetName(index)); // column name
        jsonWriter.WriteValue(values[index]); // value in column

    }
    jsonWriter.WriteEndObject();

    reader.Close();

} catch (SqlException sqlException) { // exception
    context.Response.ContentType = "text/plain";
    context.Response.Write("Connection Exception: ");
    context.Response.Write(sqlException.ToString() + "\n");
} finally {
    theSqlConnection.Close(); // close the connection
}
// END of method
// the above method returns sb and another uses it to return as HTTP Response...
StringBuilder theTicket = getInfo(context, ticketID);
context.Response.ContentType = "application/json";
context.Response.Write(theTicket);

... so the StringBuilder sb variable is the JSON object that represents the row I wanted to query. Here is the JavaScript...

$.ajax({
    type: 'GET',
    url: 'Preview.ashx',
    data: 'ticketID=' + ticketID,
    dataType: "json",
    success: function (data) {

        // data is the JSON object the server spits out
        // do stuff with the data
    }
});

Thanks to Scott for his answer which inspired me to come to my solution.

Hristo

Solution 2

My version:

This doesn't use DataSchema and also wraps the results in an array, instead of using a writer per row.

SqlDataReader rdr = cmd.ExecuteReader();

StringBuilder sb = new StringBuilder();
StringWriter sw = new StringWriter(sb);    

using (JsonWriter jsonWriter = new JsonTextWriter(sw)) 
{    
    jsonWriter.WriteStartArray();

    while (rdr.Read())
    {
        jsonWriter.WriteStartObject();

        int fields = rdr.FieldCount;

        for (int i = 0; i < fields; i++)
        { 
            jsonWriter.WritePropertyName(rdr.GetName(i));
            jsonWriter.WriteValue(rdr[i]);
        }

        jsonWriter.WriteEndObject();
    }

    jsonWriter.WriteEndArray();
}

Solution 3

EDITED FOR SPECIFIC EXAMPLE:

theSqlConnection.Open();

SqlDataReader reader = sqlCommand.ExecuteReader();
DataTable schemaTable = reader.GetSchemaTable();

foreach (DataRow row in schemaTable.Rows)
{
    StringBuilder sb = new StringBuilder();
    StringWriter sw = new StringWriter(sb);

    using (JsonWriter jsonWriter = new JsonTextWriter(sw)) 
    {    
        jsonWriter.WriteStartObject();

        foreach (DataColumn column in schemaTable.Columns)
        {
            jsonWriter.WritePropertyName(column.ColumnName);
            jsonWriter.WriteValue(row[column]);
        }

        jsonWriter.WriteEndObject();
    }
}

theSqlConnection.Close();

Comments

[Code Maverick]

Edited answer to show example off of msdn. You can also view other ways to do something similar on that page.

[Hristo]

@Scott... awesome! I'll put this together. Once I get my upvoting privileges back, I'll give you a +1

[Code Maverick]

Obviously though, from the above you can deduce how to incorporate that into your specific need.

[Hristo]

@Scott... now this leads to another question. How would I build the JsonWriter using this looping? Can I predefine the property names and then add the values as I loop through the columns?

[Code Maverick]

@Hristo - Let me edit the answer again for you. I've never used Json.NET before, but I will edit to what I would try.

[Hristo]

@Scott... I appreciate it. Take your time. I won't be working on this until Friday anyway.

[Code Maverick]

@Hristo - Updated. Not sure if that will work or if that's exactly what you want, but that's what I assume you are talking about.

[Hristo]

@Scott... ahhh cool! That looks like it should work. I'll try it out later this week when I get back to work and I'll let you know. Much appreciated!!!

[Code Maverick]

@Hristo - No problem, let me know ... I'd be interested to know as well! Good luck.

[Hristo]

@Scott... definitely. Thanks!

[Hristo]

@Scott... I implemented your code above and its not working totally right. I think the problem is the GetSchemaTable(). I don't want the whole table, just a row that the SqlReader will read. What you suggested works but its just not getting the correct row and column names

[Hristo]

@Scott... I figured it out. Check out my solution: stackoverflow.com/questions/5554472/…. Thanks for your help!

[Deep in the Code]

Very helpful! Thank you very much!

[Deep in the Code]

This answer, combined with the one at stackoverflow.com/a/14818354/2079606 has solved my problem! Thanks very much.

[mike]

Should use a StringBuilder object or something of the like, otherwise you're rebuilding the entire String and copying it over and over

[JaKXz]

If you had multiple rows in your datareader, would this give you an array of objects in your JSON response? i.e. [{ "row1-col1":"dataABC", "row1col2":"dataDEF" }, { "row2col1": "more data", "row2col2": "even more" }] - I'm somewhat surprised Microsoft hasn't provided a built in method for this in .NET 4.5+...

[THE JOATMON]

I really wish they would make @mentions dynamic.