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");

Solution 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?

Solution 2

Another solution:

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

Solution 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
}

Solution 4

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

Solution 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));

View more solutions

Comments

[Gaspa79]

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!!!

[drwatsoncode]

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.

[Ben]

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?

[Cameron MacFarland]

@Ben 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.

[KeithS]

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)

[Jim Balter]

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

[Bevan]

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.

[robert4]

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

[Robocide]

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 ?

[Przemysław Michalski]

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.

[Ali]

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

[Sid]

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

[michael.aird]

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

[Palec]

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.

[Andy]

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

[Cameron MacFarland]

@Andy What is disposing of the StreamWriter?

[Andy]

@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.

[Admin]

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."

[codekandis]

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.

[ahong]

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 @drwatsoncode mentioned.

[Cameron MacFarland]

@ahong 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.

[Suncat2000]

@Gaspa79 Perhaps you actually meant to say "switch UTF8 to UTF16." They're both Unicode.

[Gaspa79]

@Suncat2000 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 =)

[Miral]

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.

[satnhak]

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

[Métoule]

Shouldn't the StreamWriter be disposed?

[Andreas Vogl]

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.

[Matthew Lock]

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