Getting the Page URL of a File in CSOM If title & Name was different in Site Pages Library

postPageCtx.Credentials = new SharePointOnlineCredentials(un, securePwd);

var list = postPageCtx.Web.Lists.GetByTitle(pageLibName);

var item = list.GetItemById(pageItemID);

postPageCtx.Load(item);

postPageCtx.ExecuteQuery();

var fieldVals = item.FieldValues;

var postPageUrl = pageSiteUrl + "/SitePages/" + fieldVals["FileLeafRef"].ToString();

LIST OF INTERNAL NAMES FOR SHAREPOINT FIELDS

In order to reference a column or field using the SharePoint object model, you often need to know its internal name. For example, when creating a CAML query, you can specify the field on which to search by providing its internal name. As opposed to the display name, which can be changed in the UI, the internal name is immutable. While the default display name and the internal name are often identical or similar, they can also be very different. The difference isn’t always consistent. For example, spaces in the display name can be removed, such as IsCheckedoutToLocal, or replaced with the hexadecimal equivalent, such as HTML_x0020_File_x0020_Type. Furthermore, the display name can be the same for more than one fields, so the internal name is the only way to distinguish between them.

You can quickly determine the internal name of a field using the UI:

1. Open the List Settings page
2. Under the Columns section, select a column to view the Edit Column page
3. The URL of this page includes the internal name in the query string. For example, the URL for the Created By field includes the following query string List=%7BF641CEF1%2DCDE2%2D49E1%2D9800%2D861A408EF890%7D&Field=Author. The value for the Field parameter, Author, is the internal name for Created By.

However, this approach isn’t always convenient. If the column you want doesn’t already belong to the list, you’d have to first add it.

For reference purpose, below is the list of the display name and corresponding internal name for the default fields of a Document Library.

TITLE	INTERNAL NAME
Approval Status	_ModerationStatus
Approver Comments	_ModerationComments
Check In Comment	_CheckinComment
Checked Out To	CheckoutUser
Checked Out To	CheckedOutTitle
Checked Out To	LinkCheckedOutTitle
Content Type	ContentType
Content Type ID	ContentTypeId
Copy Source	_CopySource
Created	Created
Created	Created_x0020_Date
Created By	Author
Document Created By	Created_x0020_By
Document Modified By	Modified_x0020_By
Edit	Edit
Edit Menu Table End	_EditMenuTableEnd
Edit Menu Table Start	_EditMenuTableStart
Effective Permissions Mask	PermMask
Encoded Absolute URL	EncodedAbsUrl
File Size	File_x0020_Size
File Size	FileSizeDisplay
File Type	File_x0020_Type
GUID	GUID
Has Copy Destinations	_HasCopyDestinations
Html File Link	xd_ProgID
HTML File Type	HTML_x0020_File_x0020_Type
ID	ID
ID of the User who has the item Checked Out	CheckedOutUserId
Instance ID	InstanceID
Is Checked out to local	IsCheckedoutToLocal
Is Current Version	_IsCurrentVersion
Is Signed	xd_Signature
Item Type	FSObjType
Level	_Level
Merge	Combine
Modified	Modified
Modified	Last_x0020_Modified
Modified By	Editor
Name	FileLeafRef
Name	LinkFilenameNoMenu
Name	LinkFilename
Name	BaseName
Order	Order
owshiddenversion	owshiddenversion
Path	FileDirRef
ProgId	ProgId
Property Bag	MetaInfo
Relink	RepairDocument
ScopeId	ScopeId
Select	SelectTitle
Select	SelectFilename
Server Relative URL	ServerUrl
Shared File Index	_SharedFileIndex
Source Name (Converted Document)	ParentLeafName
Source Url	_SourceUrl
Source Version (Converted Document)	ParentVersionString
Template Link	TemplateUrl
Title	Title
Type	DocIcon
UI Version	_UIVersion
Unique Id	UniqueId
URL Path	FileRef
Version	_UIVersionString
Virus Status	VirusStatus
Workflow Instance ID	WorkflowInstanceID
Workflow Version	WorkflowVersion

In my case, I’m interested in the Name field. I wouldn’t have guessed that it would be mapped to FileLeafRef.

Referenced Link : 

https://blog.softartisans.com/2009/12/08/list-of-internal-names-for-sharepoint-fields/

My code for CSOM as given Below.

postPageCtx.Credentials = new SharePointOnlineCredentials(userName, secureUsrPwd);

                    var list = postPageCtx.Web.Lists.GetByTitle(pageLibName);

                    var item = list.GetItemById(pageItemID);

                    postPageCtx.Load(item);

                    postPageCtx.ExecuteQuery();

                    var fieldVals = item.FieldValues;

var postPageUrl = pageSiteUrl + "/SitePages/" + fieldVals["FileLeafRef"].ToString();