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It's one of those things I've implemented many times, usually when phrase "... and we'd also like to be able to export it as a CSV." has been tacked onto the end of some requirement at the last minute! Accordingly, each time I have had to knock up some sort of "CSV writer" at the last minute, I have invariably used a different approach.
The approach I am generally favoring at the moment, mainly for its versatility, is the use of an extension method which can be called from any strongly-typed collection. After all, in most scenarios, our data is usually in the shape of some form of object collection. In brief, the extension method is as follows:
public static void ToCsv<T>(this IEnumerable<T> objects,
Stream outputStream, Encoding encoding, char columnSeparator, string lineTerminator,
char encapsulationCharacter, bool autoGenerateColumnHeaders, string[] columnHeaders,
params Expression<Func<T, object>>[] outputValues)
{
StreamWriter writer = new StreamWriter(outputStream, encoding);
WriteColumnHeaders(writer, columnSeparator, lineTerminator,
encapsulationCharacter, autoGenerateColumnHeaders, columnHeaders, outputValues);
WriteData(objects, writer, columnSeparator, lineTerminator,
encapsulationCharacter, outputValues);
writer.Flush();
}
<Extension()>
Public Sub ToCsv(Of T)(ByVal objects As IEnumerable(Of T),
ByVal outputStream As Stream, ByVal encoding As Encoding,
ByVal columnSeparator As Char, ByVal lineTerminator As String,
ByVal encapsulationCharacter As Char, ByVal autoGenerateColumns As Boolean,
ByVal columnHeaders() As String,
ByVal ParamArray outputValues() As Expression(Of Func(Of T, Object)))
Dim writer As StreamWriter = New StreamWriter(outputStream, encoding)
WriteColumnHeaders(writer, columnSeparator, lineTerminator,
encapsulationCharacter, autoGenerateColumns, columnHeaders, outputValues)
WriteData(objects, writer, columnSeparator, lineTerminator,
encapsulationCharacter, outputValues)
writer.Flush()
End Sub
The parameters of the method are as follows:
| Parameter | Description |
objects | The collection of objects to be output in the CSV. |
outputStream | The Stream to output the CSV to. For example, this could be a filesystem stream or an HTTP stream. |
encoding | The type of character encoding to use. |
columnSeparator | The character used to separate the columns. Traditionally a comma (','). |
lineTerminator | The character sequence to denote the end of a line, e.g.: CRLF for Windows, LF for UNIX. |
encapsulationCharacter | The character used to encapsulate a value if that value contains the columnSeparator character. Traditionally double-quotes ('"'). |
autoGenerateColumnHeaders | Specifies whether to auto-generate the column headers. |
columnHeaders | Specifies column headers. Ignored if autoGenerateColumnHeaders is true. |
outputValues | A series of expressions to determine which values are to be output to the CSV. |
As you can see, the extension methods calls two methods: WriteColumnHeaders() and WriteData() for writing the column headers and data respectively. Firstly, let's look at the code for WriteColumnHeaders():
private static void WriteColumnHeaders<T>(StreamWriter writer,
char columnSeparator, string lineTerminator, char encapsulationCharacter,
bool autoGenerateColumnHeaders, string[] columnHeaders,
params Expression<Func<T, object>>[] outputValues)
{
if (autoGenerateColumnHeaders)
{
for (int i = 0; i < outputValues.Length; i++)
{
Expression<Func<T, object>> expression = outputValues[i];
string columnHeader;
if (expression.Body is MemberExpression)
{
MemberExpression body = (MemberExpression)expression.Body;
columnHeader = body.Member.Name;
}
else
columnHeader = expression.Body.ToString();
writer.Write(String.Format("{0}{1}",
columnHeader.EncapsulateIfRequired
(columnSeparator, encapsulationCharacter), columnSeparator));
}
writer.Write(lineTerminator);
}
else
{
if (columnHeaders != null && columnHeaders.Length > 0)
{
if (columnHeaders.Length == outputValues.Length)
{
for (int i = 0; i < columnHeaders.Length; i++)
writer.Write(String.Format("{0}{1}",
columnHeaders[i].EncapsulateIfRequired
(columnSeparator, encapsulationCharacter), columnSeparator));
writer.Write(lineTerminator);
}
else
throw new ArgumentException
("The number of column headers does not match the number of output values.");
}
}
}
Private Sub WriteColumnHeaders(Of T)(ByVal writer As StreamWriter, _
ByVal columnSeparator As Char, ByVal lineTerminator As String, _
ByVal encapsulationCharacter As Char, ByVal autoGenerateColumns As Boolean, _
ByVal columnHeaders() As String, ByVal ParamArray outputValues() As Expression(Of Func(Of T, Object)))
If autoGenerateColumns Then
For i As Integer = 0 To outputValues.Length - 1
Dim expression As Expression(Of Func(Of T, Object)) = outputValues(i)
Dim columnHeader As String
If TypeOf expression.Body Is MemberExpression Then
Dim body As MemberExpression = DirectCast(expression.Body, MemberExpression)
columnHeader = body.Member.Name
Else
columnHeader = expression.Body.ToString()
End If
writer.Write("{0}{1}", _
columnHeader.EncapsulateIfRequired(columnSeparator, encapsulationCharacter), columnSeparator)
Next
writer.Write(lineTerminator)
Else
If Not columnHeaders Is Nothing And columnHeaders.Length > 0 Then
If columnHeaders.Length = outputValues.Length Then
For i As Integer = 0 To columnHeaders.Length - 1
writer.Write(String.Format("{0}{1}", _
columnHeaders(i).EncapsulateIfRequired_
(columnSeparator, encapsulationCharacter), columnSeparator))
Next
writer.Write(lineTerminator)
Else
Throw New ArgumentException_
("The number of column headers does not match the number of output values.")
End If
End If
End If
End Sub
If autoGenerateColumns is set to true, then this method will evaluate each of the expressions specified in outputValues. If the expression is a simple MemberExpression (i.e.: a call to a single property) then the column header will be set to the name of the member. If not, then the column header will be set to the string form of the expression.
If autoGenerateColumns is set to false, then the method will use the column headers supplied in columnHeaders, first making sure that the number of columns and column headers match.
Next, the code for the WriteData() method:
private static void WriteData<T>(this IEnumerable<T> objects,
StreamWriter writer, char columnSeparator, string lineTerminator,
char encapsulationCharacter, params Expression<Func<T, object>>[] outputValues)
{
foreach (T obj in objects)
{
if (obj != null)
{
for (int i = 0; i < outputValues.Length; i++)
{
Func<T, object> valueFunc = outputValues[i].Compile();
object value = valueFunc(obj);
if (value != null)
{
string valueString = value.ToString();
writer.Write(valueString.EncapsulateIfRequired
(columnSeparator, encapsulationCharacter));
}
writer.Write(columnSeparator);
}
writer.Write(lineTerminator);
}
}
}
Private Sub WriteData(Of T)(ByVal objects As IEnumerable(Of T), _
ByVal writer As StreamWriter, ByVal columnSeparator As Char, _
ByVal lineTerminator As String, ByVal encapsulationCharacter As Char, _
ByVal ParamArray outputValues() As Expression(Of Func(Of T, Object)))
For Each obj As T In objects
If Not obj Is Nothing Then
For i As Integer = 0 To outputValues.Length - 1
Dim valueFunc As Func(Of T, Object) = outputValues(i).Compile()
Dim value As Object = valueFunc(obj)
If Not value Is Nothing Then
Dim valueString As String = value.ToString()
writer.Write(valueString.EncapsulateIfRequired_
(columnSeparator, encapsulationCharacter))
End If
writer.Write(columnSeparator)
Next
writer.Write(lineTerminator)
End If
Next
End Sub
This method enumerates through our collection of objects and outputs the desired values to our CSV, in the order we have specified them.
You will see that both of these methods make use of a further extension method, EncapsulateIfRequired(), for encapsulating the string values if they contain the column-separator. The code for this method is as follows:
private static string EncapsulateIfRequired
(this string theString, char columnSeparator, char encapsulationCharacter)
{
if (theString.Contains(columnSeparator))
return String.Format("{1}{0}{1}", theString, encapsulationCharacter);
else
return theString;
}
<Extension()>
Private Function EncapsulateIfRequired(ByVal theString As String, _
ByVal columnSeparator As Char, ByVal encapsulationCharacter As Char) As String
If theString.Contains(columnSeparator) Then
Return String.Format("{1}{0}{1}", theString, encapsulationCharacter)
Else
Return theString
End If
End Function
With all this in place, you can add various overloads as required, to supply default values when calling the method. For example:
public static void ToCsv<T>(this IEnumerable<T> objects,
Stream outputStream, params Expression<Func<T, object>>[] outputValues)
{
objects.ToCsv(outputStream, Encoding.Default,
',', "\r\n", '"', true, null, outputValues);
}
<Extension()>
Public Sub ToCsv(Of T)(ByVal objects As IEnumerable(Of T), _
ByVal outputStream As Stream, ByVal ParamArray outputValues() As Expression(Of Func(Of T, Object)))
objects.ToCsv(outputStream, Encoding.Default, ",", _
vbCrLf, """", True, Nothing, outputValues)
End Sub
An Example
In this example, we are going to output an array of Person objects to a CSV file on the filesystem, with the columns in the order of LastName followed by FirstName:
public class Person
{
public string FirstName { get; set; }
public string LastName { get; set; }
}
Person[] people = new Person[]
{
new Person() { FirstName = "Joe", LastName = "Bloggs" },
new Person() { FirstName = "Fred", LastName = "Bloggs" },
new Person() { FirstName = "John", LastName = "Smith" },
new Person() { FirstName= "David, John", LastName = "Jones" }
};
using (Stream fileStream = new FileStream_
("People.csv", FileMode.Create, FileAccess.Write, FileShare.None))
people.ToCsv(fileStream, x => x.LastName, x => x.FirstName);
Public Class Person
Public Property FirstName As String
Public Property LastName As String
End Class
Dim people() As Person = _
{ _
New Person() With {.FirstName = "Joe", .LastName = "Bloggs"}, _
New Person() With {.FirstName = "Fred", .LastName = "Bloggs"}, _
New Person() With {.FirstName = "John", .LastName = "Smith"}, _
New Person() With {.FirstName = "David, John", .LastName = "Jones"}
}
Using fileStream As Stream = New FileStream_
("People.csv", FileMode.Create, FileAccess.Write, FileShare.None)
people.ToCsv(fileStream, Function(x) x.LastName, Function(x) x.FirstName)
End Using
