check if a string contains anything except spaces and letters

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C#

.NET

Visual-Studio

, +

i am working on a project where i need to make validation to allow only letters and spaces
anything except that should return false.. how can i do this ?
i have my working code

bool usernamecheckchar = txboxName.Text.Any(x => !char.IsLetter(x));
            if (usernamecheckchar == true)
            {
                //entered text may contain spaces and characters only
                bool usernamecheckspaces = txboxName.Text.Contains(" ");
                if (usernamecheckspaces == true)
                {

                }
                else
                {

                }
            }
            else
            {

            }