Do you mean the standard ID3D11Buffer resource, or are you using some framework on top of D3D11?

In general, to read the contents of a buffer in D3D11, you need to use a staging buffer. This is more or less a buffer resource of the same size that is created with the staging usage flag. Then you copy the desired buffer to the staging buffer with ID3D11DeviceContext::CopyResource (or one of the other methods for copying resource memory). This staging buffer can then be mapped and written to file.

The process for writing from file to the resource depends on when you have the data from file. If you can read the file before creating the resource, then you can provide the initial data to the resource creation method directly. Otherwise, depending on the target usage of the buffer you can use update subresource, or use the staging buffer in the reverse order.

Well since you can't CPU-Read from a static (default)/dynamic buffer, you have to create a second, staging-usage buffer where you map it, read the data, and then export the data to whatever format you're writing to. Then when you load said format up, you use that data as you normally would when initializing the buffer. Staging buffers can also be used the other way too, e.g. for a default-usage buffer that you may want to write data to after its been created.

Edit: Bah! Beaten to the punch

1. Create a staging buffer with the same size
2. Copy to the staging buffer
3. Map the staging buffer to get a pointer to the buffer data
4. Write that data to a file using whatever file I/O classes/functions you're comfortable with

For any practical usage you'll probably want to also write some data indicating the size of the buffer, as well as other properties such as stride or format, the flags used to create it, etc. You'll probably want to do that before step 4, so that you have a little file header before the actual buffer data.

Reading it back in is just as easy. Read in your header data, use the header data to allocate enough memory to store the buffer data, read in the buffer data to that memory, then provide a pointer to that memory in the pSysMem pointer D3D11_SUBRESOURCE_DATA when creating the buffer with the size and flags specified by the header.

This is how I do it for my StructuredBuffer class:


struct StructuredBuffer
{
ID3D11BufferPtr Buffer;
ID3D11ShaderResourceViewPtr SRView;
ID3D11UnorderedAccessViewPtr UAView;
UINT Size;
UINT Stride;
UINT NumElements;

StructuredBuffer();

void Initialize(ID3D11Device* device, UINT stride, UINT numElements, BOOL useAsUAV = false,
BOOL appendConsume = false, BOOL useAsDrawIndirect = false, const void* initData = NULL);

void WriteToFile(const WCHAR* path, ID3D11Device* device, ID3D11DeviceContext* context);
void ReadFromFile(const WCHAR* path, ID3D11Device* device);
};

StructuredBuffer::StructuredBuffer() : Size(0), Stride(0), NumElements(0)
{
}

void StructuredBuffer::Initialize(ID3D11Device* device, UINT stride, UINT numElements, BOOL useAsUAV,
BOOL appendConsume, BOOL useAsDrawIndirect, const void* initData)
{
Size = stride * numElements;
Stride = stride;
NumElements = numElements;

if(appendConsume || useAsDrawIndirect)
useAsUAV = true;

D3D11_BUFFER_DESC bufferDesc;
bufferDesc.ByteWidth = stride * numElements;
bufferDesc.Usage = D3D11_USAGE_DEFAULT;
bufferDesc.BindFlags = D3D11_BIND_SHADER_RESOURCE;
bufferDesc.BindFlags |= useAsUAV ? D3D11_BIND_UNORDERED_ACCESS : 0;
bufferDesc.CPUAccessFlags = 0;
bufferDesc.MiscFlags = D3D11_RESOURCE_MISC_BUFFER_STRUCTURED;
bufferDesc.MiscFlags |= useAsDrawIndirect ? D3D11_RESOURCE_MISC_DRAWINDIRECT_ARGS : 0;
bufferDesc.StructureByteStride = stride;

D3D11_SUBRESOURCE_DATA subresourceData;
subresourceData.pSysMem = initData;
subresourceData.SysMemPitch = 0;
subresourceData.SysMemSlicePitch = 0;

DXCall(device->CreateBuffer(&bufferDesc, initData != NULL ? &subresourceData : NULL, &Buffer));

D3D11_SHADER_RESOURCE_VIEW_DESC srvDesc;
srvDesc.Format = DXGI_FORMAT_UNKNOWN;
srvDesc.ViewDimension = D3D11_SRV_DIMENSION_BUFFER;
srvDesc.Buffer.ElementOffset = 0;
srvDesc.Buffer.ElementWidth = numElements;
DXCall(device->CreateShaderResourceView(Buffer, &srvDesc, &SRView));

if(useAsUAV)
{
D3D11_UNORDERED_ACCESS_VIEW_DESC uavDesc;
uavDesc.Format = DXGI_FORMAT_UNKNOWN;
uavDesc.ViewDimension = D3D11_UAV_DIMENSION_BUFFER;
uavDesc.Buffer.FirstElement = 0;
uavDesc.Buffer.Flags = appendConsume ? D3D11_BUFFER_UAV_FLAG_APPEND : 0;
uavDesc.Buffer.NumElements = numElements;
DXCall(device->CreateUnorderedAccessView(Buffer, &uavDesc, &UAView));
}
}

void StructuredBuffer::WriteToFile(const WCHAR* path, ID3D11Device* device, ID3D11DeviceContext* context)
{
_ASSERT(Buffer != NULL);

// Get the buffer info
D3D11_BUFFER_DESC desc;
Buffer->GetDesc(&desc);

UINT32 useAsUAV = (desc.BindFlags & D3D11_BIND_UNORDERED_ACCESS) ? 1 : 0;
UINT32 useAsDrawIndirect = (desc.MiscFlags & D3D11_RESOURCE_MISC_DRAWINDIRECT_ARGS) ? 1 : 0;

UINT32 appendConsume = 0;
if(useAsUAV)
{
D3D11_UNORDERED_ACCESS_VIEW_DESC uavDesc;
UAView->GetDesc(&uavDesc);
appendConsume = (uavDesc.Format & D3D11_BUFFER_UAV_FLAG_APPEND) ? 1 : 0;
}

// If the exists, delete it
if(FileExists(path))
Win32Call(DeleteFile(path));

// Create the file
HANDLE fileHandle = CreateFile(path, GENERIC_WRITE, 0, NULL, CREATE_NEW, FILE_ATTRIBUTE_NORMAL, NULL);
if(fileHandle == INVALID_HANDLE_VALUE)
Win32Call(false);

// Write the buffer info
DWORD bytesWritten = 0;
Win32Call(WriteFile(fileHandle, &Size, 4, &bytesWritten, NULL));
Win32Call(WriteFile(fileHandle, &Stride, 4, &bytesWritten, NULL));
Win32Call(WriteFile(fileHandle, &NumElements, 4, &bytesWritten, NULL));
Win32Call(WriteFile(fileHandle, &useAsUAV, 4, &bytesWritten, NULL));
Win32Call(WriteFile(fileHandle, &useAsDrawIndirect, 4, &bytesWritten, NULL));
Win32Call(WriteFile(fileHandle, &appendConsume, 4, &bytesWritten, NULL));

// Get the buffer data
StagingBuffer stagingBuffer;
stagingBuffer.Initialize(device, Size);
context->CopyResource(stagingBuffer.Buffer, Buffer);
const void* bufferData= stagingBuffer.Map(context);

// Write the data to the file
Win32Call(WriteFile(fileHandle, bufferData, Size, &bytesWritten, NULL));

// Un-map the staging buffer
stagingBuffer.Unmap(context);

// Close the file
Win32Call(CloseHandle(fileHandle));
}

void StructuredBuffer::ReadFromFile(const WCHAR* path, ID3D11Device* device)
{
// Open the file
HANDLE fileHandle = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
if(fileHandle == INVALID_HANDLE_VALUE)
Win32Call(false);

// Read the buffer info
UINT32 useAsUAV, useAsDrawIndirect, appendConsume;
DWORD bytesRead = 0;
Win32Call(ReadFile(fileHandle, &Size, 4, &bytesRead, NULL));
Win32Call(ReadFile(fileHandle, &Stride, 4, &bytesRead, NULL));
Win32Call(ReadFile(fileHandle, &NumElements, 4, &bytesRead, NULL));
Win32Call(ReadFile(fileHandle, &useAsUAV, 4, &bytesRead, NULL));
Win32Call(ReadFile(fileHandle, &useAsDrawIndirect, 4, &bytesRead, NULL));
Win32Call(ReadFile(fileHandle, &appendConsume, 4, &bytesRead, NULL));

// Read the buffer data
UINT8* bufferData = new UINT8[Size];
Win32Call(ReadFile(fileHandle, bufferData, Size, &bytesRead, NULL));

// Close the file
Win32Call(CloseHandle(fileHandle));

// Init
Initialize(device, Stride, NumElements, useAsUAV, appendConsume, useAsDrawIndirect, bufferData);

// Clean up
delete [] bufferData;
}


And this is the StagingBuffer class:

struct StagingBuffer
{
ID3D11BufferPtr Buffer;
UINT Size;

StagingBuffer();

void Initialize(ID3D11Device* device, UINT size);
void* Map(ID3D11DeviceContext* context);
void Unmap(ID3D11DeviceContext* context);
};

StagingBuffer::StagingBuffer() : Size(0)
{
}

void StagingBuffer::Initialize(ID3D11Device* device, UINT size)
{
Size = size;

D3D11_BUFFER_DESC bufferDesc;
bufferDesc.ByteWidth = Size;
bufferDesc.Usage = D3D11_USAGE_STAGING;
bufferDesc.BindFlags = 0;
bufferDesc.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
bufferDesc.MiscFlags = 0;
bufferDesc.StructureByteStride = 0;
DXCall(device->CreateBuffer(&bufferDesc, NULL, &Buffer));
}

void* StagingBuffer::Map(ID3D11DeviceContext* context)
{
D3D11_MAPPED_SUBRESOURCE mapped;
DXCall(context->Map(Buffer, 0, D3D11_MAP_READ, 0, &mapped));

return mapped.pData;
}

void StagingBuffer::Unmap(ID3D11DeviceContext* context)
{
context->Unmap(Buffer, 0);
}