I came across a requirement where I needed a way to be able to retrieve pages that have been tagged with a particular Managed Metadata (MMD) value. 

Let's say there's a MMD field called Category. And the users should be able to use the Taxonomy "picker" control to select which category value they want to filter their results by.

Let's create a WebPart to do this. Use the Web Part Tool Pane EditorPart, add it to your Web Part for filtering. The EditorPart doesn't do the actual persisting of the data, so I need to create public properties on the parent Web Part that store the selected value. My web part, "MyWebPart" has the following three public properties:

The picker control should bound to the same node that my Category site column is already bound to, in the MMD tree.

Here's the code for the EditorPart class:

class Editor : EditorPart
{
private TaxonomyWebTaggingControl termPicker;

/// <summary>
/// Adds the picker control and binds it to the same term set that the
/// Category site column is bound to.
/// </summary>
protected override void CreateChildControls()
{
try
{
TaxonomyField category = SPContext.Current.Site.RootWeb.Fields["Category"] as TaxonomyField;

if (categories != null)
{
TaxonomyWebTaggingControl termPicker = new TaxonomyWebTaggingControl();
termPicker.SspId.Add(categories .SspId);
termPicker.TermSetId.Add(category.TermSetId);
termPicker.AllowFillIn = false;
termPicker.IsMulti = false;
termPicker.Width = 170;

this.Controls.Add(termPicker);
}
}
catch (Exception e)
{
//Display error message
}
}

/// <summary>
/// Takes the term that was selected by the user in the picker control, and
/// saves its properties to the parent web part's properties.
/// </summary>
/// <returns>A flag indicating whether the changes could be saved or not.</returns>
public override bool ApplyChanges()
{
try
{
termPicker = (TaxonomyWebTaggingControl)this.Controls[0];

//Evaluate if the user has picked a term
string selectedTaxValue = termPicker.Text;
int separatorIndex = selectedTaxValue.IndexOf("|");
if (selectedTaxValue != string.Empty && separatorIndex != -1)
{
//If the user has selected a term, save that term and its lookup list id (or ids, if there are child terms)
//to the parent web part. (The lookup ids are the ids of the term in the TaxonomyHiddenList in the current
//site collection.)
TaxonomyField category = SPContext.Current.Site.RootWeb.Fields["Category"] as TaxonomyField;
Guid termSetId = category.TermSetId;
TaxonomySession session = new TaxonomySession(SPContext.Current.Site);
Guid termId = new Guid(selectedTaxValue.Substring(separatorIndex + 1));
string filterTerm = selectedTaxValue.Substring(0, separatorIndex);
int[] wssIds = TaxonomyField.GetWssIdsOfTerm(SPContext.Current.Site, session.TermStores[0].Id, termSetId, termId, true, 100);
if (wssIds.Length > 0)
{
MyWebPart myWebPart = this.WebPartToEdit as MyWebPart;
myWebPart.CateogryWssIds = wssIds;
myWebPart.FilterTerm = filterTerm;
myWebPart.CategoryTermId = termId;
myWebPart.SaveChanges();
}
else
{
return false;
}
}
}
catch (Exception e)
{
return false;
}

return true;
}

/// <summary>
///Retrieves the properties stored in the web part and sets the value of the
/// metadata picker control based on them.
/// </summary>
public override void SyncChanges()
{
EnsureChildControls();

termPicker = (TaxonomyWebTaggingControl)this.Controls[0];
MyWebPart myWebPart = this.WebPartToEdit as MyWebPart;

if (myWebPart.CateogryWssIds != null &&
myWebPart.CateogryWssIds.Length != 0 &&
termPicker.Text == string.Empty)
{
termPicker.Text = String.Format("{0}|{1}", myWebPart.FilterTerm,
myWebPart.CategoryTermId.ToString());
}
}
}

Now that I have an EditorPart, I need to add it to MyWebPart. I can do it like this:

public override EditorPartCollection CreateEditorParts()
{
Editor editor = new Editor();
editor.ID = this.ID + "_Editor";
editor.Title = "Filter Term";
List editors = new List();
editors.Add(editor);
return new EditorPartCollection(editors);
}

Add the following method to make sure the changes made by the Editor part get persisted to the web part:

public void SaveChanges()
{
this.SetPersonalizationDirty();
}


When it's time to build my CAML query string, I do it like this: