SharePoint Programming

Here is a Code snippet for creating a new Site column and adding it the site collection Site column gallery and int0 a newly created content type.

public void CreateSiteColum()
{

string columnGroup = "Custom Columns";

// Adding Custom Column Amount
string amountFieldName = site.Fields.Add("Amount", SPFieldType.Currency, false);

SPFieldCurrency amountField = (SPFieldCurrency)site.Fields.GetFieldByInternalName(amountFieldName);

amountField.Group = columnGroup;
amountField.DisplayFormat = SPNumberFormatTypes.TwoDecimals;
amountField.MinimumValue = 0;
amountField.Update();

See detailed Post @ Create site column in Sharepoint 2010

/* CREATE SITE CONTENT TYPES */

string contentTypeGroup = "Custom Content Types";

// Get a Document content type to be the parent of our new custom content type.
SPContentType documentCType = site.AvailableContentTypes[SPBuiltInContentTypeId.Document];

// Create the Custom Financial Document content type.
SPContentType financialDocumentCType = new SPContentType(documentCType, site.ContentTypes, "Financial Document");
site.ContentTypes.Add(financialDocumentCType);

// Note: A content type is not initialized until after it is added.
financialDocumentCType = site.ContentTypes[financialDocumentCType.Id];
financialDocumentCType.Group = contentTypeGroup;


// Add the Amount column. Child content types inherit the column.
SPFieldLink amountFieldRef = new SPFieldLink(amountField);
financialDocumentCType.FieldLinks.Add(amountFieldRef);

// Commit changes.
financialDocumentCType.Update();

}

site in the above code is SPSite (or SiteCollection) Object.