Obtain content types:

• SPWeb.AvailableContentTypes - Gets the collection of all content type templates that apply to the current scope, including those of the current Web site, as well as any parent Web.
• SPWeb.ContentTypes - Gets the collection of content types associated with the the Web.
• SPList.ContentTypes - Gets the collection of content types associated with the list.
• SPFolder.UniqueContentTypeOrder - Gets or sets a list of the content type objects.

SPWeb web = SPContext.Current.Web;
string contentTypeName = "My Content Type";
SPList list = SPContext.Current.List;
 
// Example #1
SPContentType ct = web.AvailableContentTypes[new SPContentTypeId("0x0101")];
 
// Example #2
SPContentType ct = web.AvailableContentTypes[contentTypeName];
 
// Example #3
SPContentType ct = web.AvailableContentTypes[SPBuiltInContentTypeId.Item];
 
// Example #4
SPContentType ct = web.ContentTypes[contentTypeName];
 
// Example #5
SPContentType ct = list.ContentTypes[contentTypeName];
-or-
SPContentTypeCollection contentTypes = list.ContentTypes;
SPContentType ct = contentTypes[contentTypeName];
 
// Example #6
IList<SPContentType> folderContentTypes = list.RootFolder.UniqueContentTypeOrder;
foreach (SPContentType ct in folderContentTypes)
{
    // Use properties of ct:
    // ct.Id
    // ct.Id.IsChildOf(SPBuiltInContentTypeId.UntypedDocument);
    // ...
}

Add a content type to a list:

// 'web' is of type SPWeb
// 'ct' is of type SPContentType
SPList list = web.Lists["My Content Type"];
list.ContentTypes.Add(ct);
list.Update();

Create a content type:

// Example #1
 
// 'web' is of type SPWeb
// 'contentTypeName' is the name of the content type to create
 
if (web.AvailableContentTypes[contentTypeName] == null)
{
    // 0x0101 indicates Document Content Type
    // http://msdn.microsoft.com/en-us/library/aa543822.aspx
 
    SPContentType parentContentType = 
        web.AvailableContentTypes[new SPContentTypeId("0x0101")];
    SPContentType newContentType = 
        new SPContentType(parentContentType, web.ContentTypes, contentTypeName);
 
    web.ContentTypes.Add(newContentType);
}
 
// Example #2
 
SPContentType ct = web.ContentTypes["My Content Type"];
if (ct == null)
{
    // Obtain the parent.
    SPContentType parentContentType = web.AvailableContentTypes[SPBuiltInContentTypeId.Document];
 
    // Create the new content type.
    ct = new SPContentType(parentContentType, web.ContentTypes, "My Content Type");
    web.ContentTypes.Add(ct);
 
    // Initialize the new content type.
    ct = web.ContentTypes[ct.Id];
    ct.Group = "Custom Content Types";
 
    // Add columns. Child content types inherit the columns.
    ct.FieldLinks.Add(
        new SPFieldLink(web.Fields["MyField"])); // MyField has to exist in the web
 
    // Commit changes.
    ct.Update();
}
 
// Example #3
 
SPWeb web = SPContext.Current.Web;
 
// Obtain the parent a content.
SPContentType parentContentType = web.AvailableContentTypes[SPBuiltInContentTypeId.Item];
 
// Create the new content type.
SPContentType ct = new SPContentType(parentContentType, web.ContentTypes, "New Content Type");
web.ContentTypes.Add(ct);
 
// Initialize the new content type.
ct = web.ContentTypes[ct.Id];
ct.Group = "My Group Content Types";
 
// Add the columns. Child content types inherit the columns.
// field1 and field2 are variables of types that derive from SPField
// such as SPFieldBoolean or SPFieldText.
ct.FieldLinks.Add(new SPFieldLink(field1));
ct.FieldLinks.Add(new SPFieldLink(field2));
 
// Commit changes.
ct.Update();

Determine the parent content type:

// Example #1
SPListItem item = SPContext.Current.ListItem;
if (item != null)
{
    bool isDS = item.ContentTypeId.IsChildOf(SPBuiltInContentTypeId.DocumentSet));
    // ...
}

Add content types identified by the names 'MyContentType1' and 'MyContentType2' to the root folder of a library 'lib':

List<SPContentType> contentTypes = new List<SPContentType>();
contentTypes.Add(lib.ContentTypes["MyContentType1"]);
contentTypes.Add(lib.ContentTypes["MyContentType2"]);
lib.RootFolder.UniqueContentTypeOrder = contentTypes;
lib.RootFolder.Update();

Obtain the name of a content type in an event receiver. Return null if the name can't be determined:

private string GetContentTypeName(SPItemEventProperties properties)
{
    if (properties.AfterProperties["ContentTypeId"] != null)
    {
        string contentTypeIdStr = properties.AfterProperties["ContentTypeId"].ToString();
        SPContentTypeId contentTypeId = new SPContentTypeId(contentTypeIdStr);
        SPContentType contentType = properties.List.ContentTypes[contentTypeId];
        if (contentType != null)
        {
            string contentTypeName = contentType.Name.ToLower();
            return contentTypeName;
        }
        else
        {
            return null;
        }
    }
    else
    {
        return null;
    }
}

Make sure a library 'lib' accepts multiple content types:

if (!lib.AllowContentTypes)
{
    lib.ContentTypesEnabled = true;
    lib.Update();
}