How do I generate a stream from a string?

Question

I need to write a unit test for a method that takes a stream which comes from a text file. I would like to do do something like this:

Stream s = GenerateStreamFromString("a,b \n c,d");

Gadolinium

For memory saving solution, see StringReaderStream in https://mcmap.net/q/52888/-reading-string-as-a-stream-without-copying – Saddle

Related: Reading string as a stream without copying. – Jubilate

Answer 1

public static Stream GenerateStreamFromString(string s)
{
    var stream = new MemoryStream();
    var writer = new StreamWriter(stream);
    writer.Write(s);
    writer.Flush();
    stream.Position = 0;
    return stream;
}

Don't forget to use Using:

using (var stream = GenerateStreamFromString("a,b \n c,d"))
{
    // ... Do stuff to stream
}

---

About the StreamWriter not being disposed. StreamWriter is just a wrapper around the base stream, and doesn't use any resources that need to be disposed. The Dispose method will close the underlying Stream that StreamWriter is writing to. In this case that is the MemoryStream we want to return.

In .NET 4.5 there is now an overload for StreamWriter that keeps the underlying stream open after the writer is disposed of, but this code does the same thing and works with other versions of .NET too.

See Is there any way to close a StreamWriter without closing its BaseStream?

Maighdlin

An important point concept to point out is that a stream is composed of bytes, while a string is composed of characters. It is crucial to understand that converting a character to one or more bytes (or to a Stream as in this case) always uses (or assumes) a particular encoding. This answer, while correct in some cases, uses the Default encoding, and may not be suitable in general. Explicitly passing an Encoding to the StreamWriter constructor would make it more apparent that the author needs to consider the implications of Encoding. – Monstrous

You say "Don't forget to use the Using" for using the stream, but in your GenerateStreamFromString method you are not using the Using with the StreamWriter. Is there a reason for this? – Machute

@Machute Yes. If you dispose of the StreamWriter the underlying stream will also be closed. We don't want that. The only reason the Writer is disposable is to clean up the stream, so it is safe to ignore. – Maighdlin

It should also be noted, that the entire string is copied to a memory which may be important for large strings because now we have one extra copy in the memory. – Edwyna

I would also suggest using the AutoFlush property of StreamWriter if you need to flush multiple times after multiple calls to Write – Koons

@CameronMacFarland what is to stop the runtime from disposing of the StreamWriter (and thus closing the stream) when it goes out of scope? – Rueful

@Rueful What is disposing of the StreamWriter? – Maighdlin

@CameronMacFarland What I was thinking about is that the garbage collector could decide to collect the StreamWriter object after it has gone out of scope but while the stream is still in use, and this would in turn cause the stream to be closed. – Rueful

Why do we have to use using with your code? According to the docs: "This type [MemoryStream] implements the IDisposable interface, but does not actually have any resources to dispose. This means that disposing it by directly calling Dispose() or by using a language construct such as using (in C#) or Using (in Visual Basic) is not necessary." – Credible

Any advantage of using the StreamWriter over just encoding the string to bytes and then doing stream.Write(stringInBytes)? This would address the danger of overlooking the encoding as @Monstrous mentioned. – Jaenicke

@Jaenicke Not really. StreamWriter is probably doing what you said internally anyway. The advantage is encapsulation and simpler code, but at the cost of abstracting things like encoding away. It depends on what you're trying to achieve. – Maighdlin

The proposed solution works (as long as the stream is not accidentally closed) but it is far more complicated than necessary. The solution of @joelnet should be the accepted answer. – Autolithography

This is a 6-line method for a one-line problem. We're not paid by the line here, people. – Tibiotarsus

This should not be the accepted answer. It is long, slow and allocates a lot of memory. – Transistorize

Answer 2

Another solution:

public static MemoryStream GenerateStreamFromString(string value)
{
    return new MemoryStream(Encoding.UTF8.GetBytes(value ?? ""));
}

Eagle

Just in case someone uses this with an XML string deserialization, I had to switch UTF8 to Unicode for it to work without a flag. Great post!!! – Resistor

I like this one (with Rhyous's tweak and the trivial extra sugar for use as an extension method) better than the accepted answer; more flexible, fewer LOC and fewer objects involved (no explicit need for a StreamWriter) – Ervin

new MemoryStream(Encoding.UTF8.GetBytes("\ufeff" + (value ?? "")) if you need to have the BOM included at the beginning of the stream – Inheritable

This is very compact syntax but it's going to cause a lot of allocations of byte[] so beware in high performance code. – Hardly

This solution still left the opportunity to make the stream readonly. new MemoryStream( value, false ). You cannot make a stream readonly if you have to write it with a stream writer. – Sacrarium

@Resistor Perhaps you actually meant to say "switch UTF8 to UTF16." They're both Unicode. – Mercantile

@Mercantile Oh I know, I probably meant that I had to change the word UTF8 for the word Unicode. But who knows it was 6 years ago =) – Resistor

One caveat with this (and other answers that don't involve StreamWriter) is that they will not include the BOM preamble in the output. This is sometimes what you want, but may not be correct if you're writing the stream as a file at some point, for example. – Degroot

Answer 3

Add this to a static string utility class:

public static Stream ToStream(this string str)
{
    MemoryStream stream = new MemoryStream();
    StreamWriter writer = new StreamWriter(stream);
    writer.Write(str);
    writer.Flush();
    stream.Position = 0;
    return stream;
}

This adds an extension function so you can simply:

using (var stringStream = "My string".ToStream())
{
    // use stringStream
}

Hummer

I discovered that the returned stream gets closed (causing semi-random exceptions) when the garbage collector cleans up the StreamWriter. The fix was to use a different constructor - one that allowed me to specify leaveOpen. – Vocative

Shouldn't the StreamWriter be disposed? – Merited

The using statement will dispose the stream writer after the variable leaves scope. – Hummer

Answer 4

public Stream GenerateStreamFromString(string s)
{
    return new MemoryStream(Encoding.UTF8.GetBytes(s));
}

Egmont

Answer 5

Modernized and slightly modified version of the extension methods for ToStream:

public static Stream ToStream(this string value) => ToStream(value, Encoding.UTF8);

public static Stream ToStream(this string value, Encoding encoding) 
                          => new MemoryStream(encoding.GetBytes(value ?? string.Empty));

---

Modification as suggested in @Palec's comment of @Shaun Bowe answer.

---

Or as a one-liner (suggested by @satnhak):

public static Stream ToStream(this string value, Encoding encoding = null) 
    => new MemoryStream((encoding ?? Encoding.UTF8).GetBytes(value ?? string.Empty));

Depart

public static Stream ToStream(this string value, Encoding encoding = null) => new MemoryStream((encoding ?? Encoding.UTF8).GetBytes(value ?? string.Empty)); – Privacy

Answer 6

I used a mix of answers like this:

public static Stream ToStream(this string str, Encoding enc = null)
{
    enc = enc ?? Encoding.UTF8;
    return new MemoryStream(enc.GetBytes(str ?? ""));
}

And then I use it like this:

String someStr="This is a Test";
Encoding enc = getEncodingFromSomeWhere();
using (Stream stream = someStr.ToStream(enc))
{
    // Do something with the stream....
}

Obtain

Thomas,why down vote ? enc= enc ?? Encoding.UTF8 allows me to specifically ask stream with specific encoding , or a default of UTF8 , and because in .net(as far i use it .net 4.0 ) you can't give a reference type other than string a default value in function signature this line is necessary, does that make sense ? – Obtain

mentioning that you need to put this in a separate class (non generic static class?) is also helpful and reduce the down votes. – Eversole

could it further be reduced to this? public static Stream ToStream(this string str, Encoding enc = Encoding.UTF8) { return new MemoryStream(enc.GetBytes(str ?? "")); } – Zirkle

Answer 7

Use the MemoryStream class, calling Encoding.GetBytes to turn your string into an array of bytes first.

Do you subsequently need a TextReader on the stream? If so, you could supply a StringReader directly, and bypass the MemoryStream and Encoding steps.

Milton

Answer 8

We use the extension methods listed below. I think you should make the developer make a decision about the encoding, so there is less magic involved.

public static class StringExtensions {

    public static Stream ToStream(this string s) {
        return s.ToStream(Encoding.UTF8);
    }

    public static Stream ToStream(this string s, Encoding encoding) {
        return new MemoryStream(encoding.GetBytes(s ?? ""));
    }
}

Thistledown

I would prefer to implement the first method as return ToStream(s, Encoding.UTF8);. In the current implementation (return s.ToStream(Encoding.UTF8);, the developer is forced to think harder to grasp the code and it seems that the case of s == null is unhandled and throws NullReferenceException. – Apotheosize

Answer 9

If you need to change the encoding I vote for @ShaunBowe's solution. But every answer here copies the whole string in memory at least once. The answers with ToCharArray + BlockCopy combo do it twice.

If that matters here is a simple Stream wrapper for the raw UTF-16 string. If used with a StreamReader select Encoding.Unicode for it:

public class StringStream : Stream
{
    private readonly string str;

    public override bool CanRead => true;
    public override bool CanSeek => true;
    public override bool CanWrite => false;
    public override long Length => str.Length * 2;
    public override long Position { get; set; } // TODO: bounds check

    public StringStream(string s) => str = s ?? throw new ArgumentNullException(nameof(s));

    public override long Seek(long offset, SeekOrigin origin)
    {
        switch (origin)
        {
            case SeekOrigin.Begin:
                Position = offset;
                break;
            case SeekOrigin.Current:
                Position += offset;
                break;
            case SeekOrigin.End:
                Position = Length - offset;
                break;
        }

        return Position;
    }

    private byte this[int i] => (i & 1) == 0 ? (byte)(str[i / 2] & 0xFF) : (byte)(str[i / 2] >> 8);

    public override int Read(byte[] buffer, int offset, int count)
    {
        // TODO: bounds check
        var len = Math.Min(count, Length - Position);
        for (int i = 0; i < len; i++)
            buffer[offset++] = this[(int)(Position++)];
        return (int)len;
    }

    public override int ReadByte() => Position >= Length ? -1 : this[(int)Position++];
    public override void Flush() { }
    public override void SetLength(long value) => throw new NotSupportedException();
    public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
    public override string ToString() => str; // ;)     
}

And here is a more complete solution with necessary bound checks (derived from MemoryStream so it has ToArray and WriteTo methods as well).

Thunderhead

Answer 10

I think you can benefit from using a MemoryStream. You can fill it with the string bytes that you obtain by using the GetBytes method of the Encoding class.

Twum

Answer 11

Here you go:

private Stream GenerateStreamFromString(String p)
{
    Byte[] bytes = UTF8Encoding.GetBytes(p);
    MemoryStream strm = new MemoryStream();
    strm.Write(bytes, 0, bytes.Length);
    return strm;
}

Educationist

The position needs to be reset after writing. Better to use the constructor, as in joelnet's answer. – Euniceeunuch

Answer 12

Now with C# 11, we can do this in one line:

var ms = new MemoryStream("some string"u8.ToArray());

Details about Utf8 Strings Literals can be found here https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-11.0/utf8-string-literals#detailed-design

Hawaiian

Answer 13

A good combination of String extensions:

public static byte[] GetBytes(this string str)
{
    byte[] bytes = new byte[str.Length * sizeof(char)];
    System.Buffer.BlockCopy(str.ToCharArray(), 0, bytes, 0, bytes.Length);
    return bytes;
}

public static Stream ToStream(this string str)
{
    Stream stringStream = new MemoryStream();
    stringStream.Read(str.GetBytes(), 0, str.Length);
    return stringStream;
}

Manon