How do I generate a stream from a string?

Another solution:

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

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
}

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

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));

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....
}

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.

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 ?? ""));
    }
}

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).

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

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.

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;
}

Using a memorystream which can be done via extension methods.

public static class StreamExtensions {
    public static Stream ToStream(this string @this, Encoding encoding) => new MemoryStream(encoding.GetBytes(@this));
    public static Stream ToStream(this string @this) => new MemoryStream(Encoding.UTF8.GetBytes(@this));
}

It is also possible to instead have a Disposable building block that uses functional programming. Instead of returning a memory stream, instead have a byte array returned and close the memory stream as soon as possible.

Using this approach we can have an extension method:

public static class Disposable {    
    public static TResult Using<TDisposable, TResult>(Func<TDisposable> factory, Func<TDisposable, TResult> fn) where TDisposable : IDisposable {
        using (var disp = factory()){
            return fn(disp);            
        }
    }
}

And some demo usage here :

string someLatin = "Si vis pacem para bellum";

var bytes = Disposable.Using(() => new MemoryStream(), mem => { mem.Write(Encoding.UTF8.GetBytes(someLatin)); return mem.ToArray(); });
Console.WriteLine(Encoding.UTF8.GetString(bytes.ToArray()));

Your tests will then have a byte array and have less side effects to worry about such as a stream lying around and must be disposed later. This might not be an option if you are going to specifically do some particular stream operations further down into your tests besides just reading out the byte array from a stream.

If you don't care about encoding, this is a stream wrapper with zero allocation using spans. Also, the block copy has no branching.

This is written in dotnet 8 with c# 12, so you may need to adapt a bit if using a lower version.

class StringStream(string @string) : Stream
{
    private readonly string _string = @string;
    private int _position = 0;

    public override bool CanRead => true;
    public override bool CanSeek => true;
    public override bool CanWrite => false;
    public override long Length => _string.Length * sizeof(char);
    public override long Position
    {
        get => _position;
        set
        {
            ArgumentOutOfRangeException.ThrowIfGreaterThan(value, Length);
            _position = (int)value;
        }
    }

    public override void Flush() { }

    public override int Read(byte[] buffer, int offset, int count)
    {
        if (_position >= Length)
        {
            return 0;
        }
        int nbytes = ReadWindow(buffer.AsSpan(offset, count));
        _position += nbytes;
        return nbytes;
    }

    private int ReadWindow(Span<byte> buffer)
    {
        int nbytes = buffer.Length;
        if (_position + nbytes > Length)
        {
            nbytes = (int)Length - _position;
        }
        if (nbytes <= 0)
        {
            // No data left
            return 0;
        }
        int alignSkew = _position & 1;
        var strPos = (_position - alignSkew) / sizeof(char);
        var window = _string.AsSpan(strPos);
        var windowBytes = MemoryMarshal.AsBytes(window).Slice(alignSkew, nbytes);
        windowBytes.CopyTo(buffer);
        return windowBytes.Length;
    }

    public override long Seek(long offset, SeekOrigin origin)
    {
        long newPosition = origin switch
        {
            SeekOrigin.Begin => offset,
            SeekOrigin.Current => _position + offset,
            SeekOrigin.End => Length + offset,
            _ => throw new ArgumentOutOfRangeException(nameof(origin))
        };
        if (newPosition < 0 || newPosition > Length)
        {
            throw new ArgumentOutOfRangeException(nameof(offset));
        }
        _position = (int)newPosition;
        return _position;
    }

    public override void SetLength(long value)
    {
        throw new NotSupportedException();
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        throw new NotSupportedException();
    }
}

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;
}